How can I win a play fight with somebody who is stronger?

I am 13 years old and me and my friend often have little play fights for fun, and he always beats me as he is a lot stronger and faster than me, and he puts a lot more force into the fight. We never punch or kick, it is mainly just grappling and twisting, which he is much better at than me. I am wondering, are there any great ways to escape from locks or holds and how can I escape when he has me on the floor with him on top, are there any moves that don't hurt the opponent too much and how can I win him in fights most of the time. I need things that are quick and easy to learn, nothing like karate, and any tips for grappling. Thanks a lot for the tips!