First of all, his primary skills are fantastic start. They're the highest they could be as a rookie. However, it sucks he has 3 atrocious in addition to very low FT.
1. Your plan sounds good. Train RB, IS and ID, and keep repeating it. But at some point, you'll need to train passing and handling, and if you have time, OD as well. Otherwise he'll be a monoskilled monster and his salary will "sky-rocket". Also you should train FT and stamina as well.
2. Getting lvl 5/6 trainer wil increase the rate of training, but not by very much. I heard that lvl 5 trainer will give a player extra 2-3 pops over 2 seasons, so really, it's not much. But if you have a lot of money saved, you could get lvl 5/6 trainer.
3. Potential doesn't really matter for U21 NT team (unless a player has like 6th man potential). All star potential is good enough for U21 NT team.
Good luck, you've got a very nice rookie there, don't spoil him.