Very disturbing Sols, I've only just picked up on thread, and given it 24 hrs before replying.
First things first; your current well being?. Your right to be concerned about celebral haematoma, they can occur 2 or even up to 3 weeks after impact, and come on very quickly. You need to get that checked out, the chances are that you're ok, but you won't know about fractures or any weakening of the skull. What you tell a hospital won't necessarily matter, although they occasionally make mistakes they see these type of injuries, and tend to know how they are caused.
The second of course of action I fear you're going to have to take is clearly much more difficult and is tantamount to one one of those life defining moments I'm afraid?. My gut reaction was probably not to far removed from your own and that of your mothers given that this is clearly a horrible lose/ lose situation. There's some very important factors you need to consider though, and when taken together they do indeed point you more or less in one direction i'm afraid.
In the first case you need to consider that your life is probably in danger. I hope this isn't alarmist or melodramatic, but alcohol and violence is a potent cocktail, and if you're unable to defend yourself I fear this is probably the grim reality/ worse case scenario you need to front up to?. Few true alcoholics are capable of reforming (I use the word advisedly as it is abused to describe any number of dependent conditions). Some recover, and some manage to arrive at a coping situation, even then the spectre is a stalking shadow, and rarely truly leaves a person. The balance of likelihoods however is that he won't seek assistance nor accept his need to, and certainly not in the short term (most alcoholics need to WANT to confront their demons and beat them) he seems a long way from that, and you don't appear to have time on your side in this one.
My experience of this kind of thing is that it involves a series of taboos which even alcoholics don't normally cross, the moment they do however, the chances of a repetition or (God forbid) escalation increase, and again I'm afraid I think it would be prudent to face up to this likelihood. The fact that he's telling his friends is of grave concern and I wouldn't under estimate its significance. Normally someone would seek to surpress this kind of thing, not brag about it as no one I know would ever where it as a badge of honour. It also tells me that his peer group are incapable of challenging him, and so another avenue of support/ corrective action is probably shut down? I suspect he might very well be the dominant 'alpha male' for want of a crasser way of putting it within this group? Similarly, I'm sure your mother is more than aware that she can't protect you, and that this is actually eating her up inwardly? It strikes me that she's probably scared too, and would desperately want to resolve the situation but doesn't know how to and might be to fearful to try? I feel this might be one of the hardest situations for you to resolve of all, as a sense of abandonment is bound the gnaw away at you, if you perceive yourself to have left her with him. Her own self-esteem is probably pretty low at the moment as she can be far from happy about what she'll probably blame herself to some extent for presciding over. It's my guess she'd probably give serious thought to running off with you if she could? I fear that burying ones head in the sand and hoping it'll go away though, isn't an answer.
Once the physical side of the pain is subsides, the mental side takes a grip, and this really can take its toll, as you never know what person will walk through the front door, or what minor action of seemingly the most innocious nature can provoke another attack. Essentially, you end up sentancing yourself to a day to day existance based around surviving, against a backdrop of an ever attendant threat which for the most part is camoflagued. Basically, its a torture which given that it can apparently strike from no where with not even any discernable element of provocation or sense of prediction makes it all the worse, and under normal circumstances will eventually take its toll on your own mental health and quality of life etc. You'll unfortunately find yourself treading on egg shells constantly wondering whether anything you do or say is likely to precipitate another assualt. It can become all consuming, and very quickly you can find its taken your own life over, as you primarily are occupying yourself with such thoughts to the level where its just about the only thing you're considering day in, day out. It's no way to carry on.
Unfortunately, I fear I have to share with you my humble opinion (which taken from a distance might be off the mark) but I'm afraid that something will cause him to snap again, and the Rubicon has been crossed once, and so presents less of barrier to do it a second time now. It might not necessarily be you that is the subject of the next assault, but you need to consider that it might be, (I believe people tend to attack the people they've attacked previuously?) and that it could be worse this time.
I'm making some assumptions here which might impact on which way you go. Am I correct in saying you're based in Ireland? would that be rural Ireland or big City Ireland? I don't know how the Garda approach these things, I believe the UK police have got better though? Am I also correct in thinking you're in your early 20's? single? and not necessarily financially independent? I'm sorry if this sounds a bit personal, but its just trying to establish what options you might have open to you. As I said earlier most of us have to face up to life defining decisions, and severing a family link isn't taken lightly, especially if you still percieve your mother to be a hostage to the situation. I think you need to balance this though against the very serious danger you might be in, and it isn't enough to content yourself with on 99.9% of the occasions its ok etc 0.01% is all it takes.
Essentially the environment isn't save for you to remain in I believe?, and something has to give. I would seriously consider taking Diminuedo up on her offer to allow yourself a time to reflect and consider your best options etc, as well as providing a haven for you to recover from the ordeal. People don't make kind offers for the sake of their own vanity I believe, and I'd have no compunction about asking for help thus if I were in your position, and someone had offered it.
Ultimately, I suspect that you and your mother will have to put your trust in the authorities, but this could leave you open to a revenge attack if they fail to prosecute it effectively, and although it hasn't been said, I'd be surprised if this wasn't at the back of your thinking?, especially as the person knows so much about you, and could probably find you, if resourceful enough? It's a horrible situation and I'm afraid there aren't any easy answers which don't involve a lot of pain I fear, and I'm sorry I can't be more upbeat. The attack sounds a little bit more than what I'll euphamistically call a 'slap'. It is dangerous, and potentially much worse, and I hope you can reconcile this in deciding what to do
Hope you're ok