I agree. The AHL would be the best place for him next year, and it's a shame he can't play there. Juniors will likely be too easy, and you don't want to see him pick up bad habits on things that he can get away in juniors, but won't fly in the bigs. But getting bottom pairing minutes in the NHL won't help him much and he's at the age where you want him to see as much ice as possible. Plus, in the AHL he could get the occasional call up for games here and there, and get better acquainted with the speed of the NHL game.
I guess I just think it's very risky having him play in the NHL next season. You only have a ten game window to return him to juniors, which might be enough time to figure it out, but if you pass that window, and he later shows he's not ready, you can't really do anything about it and it's a season of development potentially lost. The best thing might be to play him for 9 games in the NHL, then return him to juniors and let him dominate for a year.