Oh poor Dougal.
I think your vet is right to try and see what his immune system does on it's own for awhile. When winter sets in it will give him a knock and it would be good to have something of a secret weapon in hand to use if needed, rather than taking a high dose all the time.
I hear great things about flax seed oil for cats, and oddly enough am going to give to one of mine who has constant sinus trouble, and coughs from having had cat flu young.
I have experience of it with humans, good if you are having chemo, a friend who had breast cancer took it and her skin and hair looked good even during treatment, side effects seemed less, and her liver inflammation vanished! They warned her she would be very tired during treatment but she was not.
There is controversy about whether a cat can digest it, but I think they can, you can read all of the info on the net, it's freely there.
My homepathic vet also recommends fish oil for cats as well.
Discuss with your vet and see what they say - do not want any inter-reactions that would be bad for him.
We can compare notes as to how the cats do on this, but I think it takes at least 3 weeks for any difference to become visible.