Only managers in the tier above can buy him, for them he would be a low level 5 star 1 day left of this season, then he'll be a low level 4 star next season and 20yo.
If you wait till next season and sell him he'll be worth less, but he'll be a 5 star player. Everyone wanting to build up their team will no doubt want to snap him up.
So it's risky, he might devalue but sell for more with the competition to buy him, or you might get lucky and get a buyer for him just now, which you probably will as he has a special skill.
So I personally would try and sell him just now, if he is like you say a very slow trainer.