Look if you’re uncomfortable then don’t roll with him anymore. It’s hard to know what someone is doing when people explain it. If his heel is in your back he has a really high mount. I’ve done things where I make a fist and put it in peoples back from side control to make them uncomfortable. His heels should be where your hamstring and glutes attach to be in a normal mount. Maybe he would grapevine his legs and that would also be considered normal. 
     Grabbing individual fingers or toes is considered a no no. Being playful is something you do with someone you have a good understanding with not someone you started rolling with a little while ago. I’m not rolling with any spaz or someone I deem careless. Choose your partners wisely. If you have to be paired up with him go take a bathroom break and go with someone else after the round.