You are going to need to have a serious talk about consent and proper touching with him.

You have to stop this now before it does turns into a real problem later on.

Also I have to ask from personal experience so please don't take this the wrong way. Has he been around anyone who might have potentially "Tickled" him in private areas?

I ask because as a small child, that's how I was abused, and I tried to do the same thing to other kids "Playing a game" not realizing what I was doing was wrong, and later as a preteen and teen, I was one of those boys who didn't respect "No" and would try to touch and fill up girls. It took a talking to from a cop for me to start to see how serious my actions were, but it wasn't until later in life I realized everything tracked back to that abuse.

I would speak to him about consent and body autonomy, reminding him that no one has the right to touch him and he doesn't have the right to touch anyone else. I would also set the standard that tickling is NOT to be done at school.

As for the bullying, same thing body autonomy, no one has the right to touch him. He needs to stand up for himself, speak loudly and assertively saying "NO" to the bully and what he won't allow. Make a scene and get teachers involved. He also should try to set towards the front of the bus closer to the driver until this stuff stops.