I think that he is probably fast but likely not a very fast trainer, and he had already reached 6 star.
My team is one level lower. I have a GK, age 18, 152Q (5star), value 111M. He gains 16.8 progress in average for light sketching. He is quite good but not a very fast trainer. If i just buy it now and its quality is 154Q, I believe he trains more slowly.
Another player is a MC, age 20, 153Q, value 110M. His gains almost exactly the same progress on training as GK mentioned above. So I actually cannot expect player training progress. Sorry, just ignore me.