I agree with Sofia Zampelli, because I am Sofia Zampelli.
I have two accounts.
One thing that helped my friend with this problem, was she put a crop in between her lower leg and the horse and it helped her overcome this problem. Do not do this exercise over fences.
Learn to adapt to the way he jumps. Look at where and how you can hold on better with your legs on the saddle. Also do exercises to strengthen your legs that will help keep them in place. Practicing with no stirrups or bareback is also a good exercise to help with keeping your legs in the right place and to strengthen your legs.
Hope this helps💜