Get his ears checked too maybe - if sucking hurts he may be being naughty.
Period coming back should lead to "funny" tasting milk and sometimes slower letdowns - but he's still drinking the EBM, so the taste can't be too bad, and you say that expressing until the let down has happened doesn't help either, so that probably isn't the reason.
From an attachment point of view you want him to be wide open and ready - because the more he has in his mouth the less the dainty bits get bitten. So while it is very counterintuitive, one way of easing biting is to stuff more in!
Have you tried food first and then boob - so he's done some chewing and maybe got that out of his system (and I am clutching at straws here!).