you have to see what the market is buying....
its a dilemma as i have no idea what his value is but a HofF young with decent skills is going to be big $$$.
Now a lot of big teams wont buy because skill wise he wont be good enough to break into somebodies 1st team yet so it may be wiser to keep training..
also having done the hard work and trained him from lets say draftee to $2.5mil.... i bet if you were patient and trained him 1 more season he could be worth 5-6mil.... so the training profit this season is likely to be far greater than selling now and then starting the process again..
so much depends on the position of your team vs your peers... so you have to judge whats best for your franchise!