That happened to me me when I was around 7 YEARS OLD. NOBODY EVER TOUCH ME. I just figure it out by myself, playing with my brothers. Suddenly It felt good somehow rubbing during playtime. I think it is a natural way to find out about sex. You just need to talk to them explaining what is right or wrong about that, that was my mom did to me and it worked, never happened again.
If your older son just made up this game, that is one thing. It's another if the same thing is being done to him by another kid or an adult. That is why I recommended finding out how he learned to do it. That said, I still wouldn't let him dominate the younger boy in such an overtly sexual way. It's one thing to wrestle, it's another to simulate sexual activity.
There's no reason to protect them. Boys tend to play like this often. I'm sure its something ur oldest came up with. I have 3 boys and a bro. Boys will be boys. Maybe one day ur son stumbled upon it,like the were wreslting or something and he realized it felt good. It doesn't take much for kids once it feels good. If he's not being sexually abused of anything, I wouldn't stress it, he is probably just being a lil boy. Curiosity is how they learn. Do talk to him and make sure he understands what he did has to wait till he's an adult and also make sure u tell him its not ok to do that stuff with his lil bro. Good luck!
The older boy might be getting this treatment from someone older than himself. I would try to find out who taught him this behavior, without raising his concern that somehow he himself is in trouble.
Of course, after finding this out, I would also have the talk with him that this is not appropriate to do to a child and a sibling, and not appropriate to do until he is older, and all of that. When you think he gets that (again, without shaming him or anger), then watch the two of them together and be sure the message has sunk in.
If you find he was learning this from another kid, you need to tell the kid's parent. If it was from an adult, you need to contact the police.
I'm sorry if this leads in a direction that causes hassles and problems, but you must protect your kids, both of them.