I don't think it's impossible. But it depends on him and his muscle memory, and what level he is at.
We had a kid broke her arm, twice in 3 months. She had to take a year off while she underwent investigations for bone disease which might mean she breaks too easily. Once she was cleared she came back to gym and gradually built back up, but was at her old skill level in a few weeks and had caught up with her teammates within another year. I gave up gym age 11, and went back for fun after two or three years. I hadn't lost any skills at all- a couple of apparatus skills like BHS on beam needed a bit of working up for confidence, but tumbling and floor skills were all still there.
But then I know a swim coach who has had regional champion swimmers give up at 16, come back at 19/20 for fitness, and have practically forgotten how to swim!
If he is the type whose body doesn't forget, it's going to mainly be the loss of condition that makes the biggest difference. If he can keep generally fit and conditioned, that will be the biggest help to getting back in to it. Otherwise he's probably going to need an intensive 6-8 week strength and conditioning program before he starts back with skills.