I'd like to see you try and sing 'Land of Hope and Glory'! - it ain't easy
To be honest his republican credentials are well documented, so I'd tend to the view he'd look a bigger hypocrite if he suddenly started doing so. Quite apart from it being a crap song anyway. In fact I'm sure it started out as a rallying call sung at the close of London musical halls in 1745 against the Jacobite rebellion, hence the verse which they've airbrushed about "rebellious Scots to crush". Consequently it was adopted, although there is a theory that it was subversively satirical (God save the King - because only he can etc) and then goes onto describe the King by all the things he wasn't and that very few held him to be (noble, gracious etc). It's not a particularly pleasent song
I should note that there is a current English cricket captain who you were heaping praise on the other day who also refuses to sing it. Perhaps you might like to condemn him too?
As regards Jerusalem, that (strangely enough) has a much deeper association with socialism and ought to be more acceptable to him. I think you'd be very hard pressed to argue that William Blake was some kind of patriotic nationalist extolling the virtues of Britain and establishment in particular which he regarded as self indulgently corrupt and immoral. Indeed he was tried for treason (and acquitted) for allegedly having said "damn the king"