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?