I had a simillar problem with 1 of my players. Being only 6'3 with very good inside skills, decent outside. I have also an other 4 trainees at PF/C. I trained him for 2 seasons as a big man (ID, IS, R), and then i advertise him, for managers that they want to create a great SF, so now the next manager is training is guard skills. Some times is better to sell a player to an other manager, just to help him to rich his potential.