He should be fine eating adult dry food. He's getting whatever extra nutrition he needs from the wet kitten food. There shouldn't be anything harmful to him in the adult dry food. Years ago there weren't so many choices in cat food. Plenty of kittens grew up eating nothing but cat food that...
I've gotten kittens from Breeders, who I would deem responsible, at 9 weeks old. I really think it's a matter of opinion on weening from 8-16 weeks old. Lots of feral cats are weened at even earlier ages and do just fine.
Recently I purchased two Savannah kittens. One was 16 weeks and one 9...