It really depends on what you want to do with your team. If your plan was to train guards, then I would encourage you to sell him. But if you have no training going on, I think he may be more valuable if you keep him. With some training he can easily dominate in a div. V and help you improve as a team.
Giving him at least 48 mins of action is crucial for full training. You should also get a decent coach (maybe level 4? I don't know how much money you make), but avoid overpaying. And also, find one or more other trainees to train alongside him. You may train your frontline, and then sell one of the trainees to improve other positions