we tried everything with dd2, eventually we had to hold her down Then one day i said 'oh what's that? Is that some strawberry on your tooth? Let me clean that off... 'end of problem. Once she understood WHY we were brushing, she let us do it.
But one question Op, if his gums bled too easily, is it possible its hurting him to have it brushed? Might be worth a visit to a child friendly dentist just in case.