OP, could be wrong, but I think the best you'll get is a horizontal top tube one. I guess you'll need a pretty exact frame size so as to be not too small for him and not too big for you... so the very best of luck with your search!
It may also matter whether the lad's additional height is in his legs or his torso - but either way it could be awkward. If he's longer legged, just raising the seat would result in him cycling nose-down (not very comfortable); if he has a long body, he'll just end up with his chin on your back.
You could get different handlebars so that he can be more upright - we put some multi-position ones on for me (stoker) when I had back problems, angled upwards. It might not be his favoured riding position, but maybe better than headbutting you?