Iron Man is right. If he is high on your chest use your elbows to slide yourself up. Doing that remember you are trying to move yourself not him. If you move him that is a plus.
The best mount escape for me is bucking and driving up and out with your hands on his hips. It has to be fast and sometimes takes 2 back to back tries. If he is new he probably hasn't learned good balance from the mount. I don't have a video because youtube is blocked.
Credit to M.C !!