K.
I must say I do not have this problem with my son, he is almost 13 months. But my highchair wont allow him to even move that much. Are there straps on the chair to hold him in? Although I do not use them on my highchair, there are actual shoulder harnests to prevent climbing out. I have used a booster seat when I am away from home and my son does well in it, but I do strap him in.
Wish you luck?
K.