My best suggestion would be to only leave down the adult food, and give the young one some kitten food at some point during the day when you are there to supervise him eating. That way he can eat any time (the adult food) the older cat can't get into the kitten food, but at the same time the younger one has the benefit of a small daily portion of the kitten food.

Now, if all that is impossible, he is old enough for adult food. (I just think they SHOULD have it a little longer)