From now until he is 21, when you train his inside skills, you only have ID and REB. Since his IS is already at the level of a typical U21 big, he doesn't need to be trained in it at the moment.
If you have time to squeeze it in, I would also train 1v1 and PA to raise his HA and DR. The best way to train PA is by playing him 48 minutes at PG. PA trains fast on everyone, so he should pop quickly. I recommend training it until he hits inept.
For 1v1, you could get him 48 minutes at PG, SG, or both of them combined. I also advise you to do 1v1 Guards instead of Forwards. The reason why is that 1v1 Forwards trains IS a bit, and if his sublevels are high enough, he might pop in IS. Couple that with you training his ID/REB and that pop might make him a lot more expensive.