I'd be asking DS what he thinks and looking at what the school offers as a whole. Is the education good? Do you rely on its before/after school care and is that good if so? Are standards of behaviour acceptable, have they a robust and effective anti-bullying procedure? Is your son safe, nurtured and cared for at the current school?
Ask yourself where DS would go if not at this school, realistically, remembering that if you're in England most of the more desirable schools will be full and places hard to come by. Identify a school that you like with places before making any decisions and consider transport, uniform expense and the like.
How will DS cope with a move? Will he miss favourite teachers or friends in other years? Yes he may be the only lad in his class but he may have girl friends or pals in higher/lower years. How old is he - if he's due to go up to senior school in a year or two is it wise to move him now?
Remember that although his situation is unusual it isn't unique and just as HE families do you may find that you can redress the social balance by involving him in out of school activities, be that an after school club with both girls and boys or scouts/cubs etc. (Or indeed finding some other families, perhaps HE ones, to mix with out of school).
There would have to be other things "wrong" with the school before I moved my child... I hope that this might give you something to consider when making a decision.