Either or really

Just remember that he will still pop in training just not as quick.
I know some trainers who stop training guys at 22-23 but I say why?! You still have a lot more you can get out of them.
I found that have 3 trainees whilst possible, one 1 position training takes A LOT and by a lot I mean a FUCKING BITCH LOAD of management to do effectively. You have to make sure you have a scrim each week plus you set up your line up's correctly etc
I would recommend only have 2 trainees. Easier to manager and actually look after.
Only sell him if you need the money.
I found that players with Sensational in their main stat's performance pretty well in DIV III so maybe keep him as a backup for those games you want to be competitive in. Because remember you trainees may not be effective for 2 seasons after you buy them.
What you do though is up to you. You have to make the decision to rebuild with or without him.
When the Beasts Roar. Fear is struck into the hearts of all who hears - Neeber