It's happening because he's struggling to find his balance. His instructor is (I guess) concentrating on getting him to go with the forward motion - really really important as he develops as a rider. I should think she will come back to correcting his position later when she feels he's better balanced.
The alternative is to develop a seat by walking for months and months with constant work on the position at the end of the lunge line - not many kids will put up with that .
My DS started lessons a few weeks ago. I think he's been about 5 or 6 times now. He loves it and initially I was impressed as they got him trotting on his first lesson. Which with a boy is a good thing I suppose to keep them keen. He now wants to canter! LOL.
Anyway my gripe is that he has a really great position and sits really well in walk, legs in a great position etc. However in rising trot he shoots his legs forward (not quite sure how he does this and manages to rise at all but he does!) and then he struggles to rise, I mean finds it hard to go up and down in a rhythm. Who wouldn't with their legs shoved forward?! I keep telling him to put his legs back but obviously he doesn't take any notice of me! Why is his teacher not correcting him? This has been going on for weeks and the teacher has never said a thing. She keeps praising him for keeping the pony going etc and for stopping etc but not helping with the technique. I feel so frustrated and I want to say something but don't feel I can without interfering. Any suggestions?