If you don't have a big reserve team play him in U18's and if the reserve team needs more players they will take the best players from your U18's squad. This is what i did with Bojan in fm 2007 and he ripped up the U18 games and still made the starting reserve side (alongside Fowler, now thats an impressive attack!)
leave him in u18's for now. as charlo116 says, reserves will take players from the u18's if theydont have a big enough squad. what i tend to do is, as soon as they hit 17 andget on professional terms, then they go up into the reserves squad, to get on full-time training asap
Problem with either of these teams (U18 and reserves) is that the level of games is rather poor. I notice that a player in these teams rarely improves much but playing him in your first team will handicap your results.