You say DS is also doing Theory: the most important thing is not just being taught and 'learning' theory, but UNDERSTANDING it. I don't know how well the average piano teacher (no matter how good at Piano) will teach Theory or Composition.
I have looked back over some of your previous queries, going back two years or more, and I gather you are not in the UK. Why are you SO keen he learns composition? Music colleges in the UK will teach composition, but usually - I would guess - to much older students, who were Grade 8 or higher, and whose musical skills and knowledge were approaching professional levels.
Composition and Theory rely, not just on 'musical' factors, but also on the Laws of Physics, as chords and harmony observe such laws, and also serious levels of Mathematics.
In the UK the BBC Proms is currently in progress, so if it is possible I would suggest DS listen to all the ones he can, and record all the ones that are broadcast on TV. (All the concerts are on BBC Radio 3, and many may be on Internet radio.)
If you can tell me why you are so keen for him to progress and learn about Composition, then I may be able to suggest the best ways to start to achieve that. In the meantime, I would suggest when DS listens to any new music, that he tries to sing along with, and try to guess or work out what may be coming next in the piece.