At this point, I would write a letter to the principal that is short, polite and direct, saying that you understand the incident has already been dealt with but in the future, you want to be notified directly if there are any similar "hands-on" incidents on the bus or anywhere else, and you want to be notified the day of the incident. Don't get lengthy or angry; that will only cause the principal, possibly, to label you as "one of those troublesome parents" who then get ignored. Sadly, that does happen. I suggest sending an e-mail but also a letter, and ask for a reply acknowledging receipt of your letter. You should have the letter and the reply filed where you can get to them if something else happens. Let's hope you never need them.
Also, I bet you do this already, but reinforce with your son that it's OK to yell, "Don't touch me! You are not allowed to touch me!" ANY time or ANY place ANYone is touching him. Not just adults, but kids too, even kids he's afraid of at that moment. That would alert the bus driver or teacher, and certainly might get the kid who's harming him to stop, if your son is very loud and firm. Rehearse it with him so he hears his own yelliing voice. We tell kids not to yell all the time -- so it helps to tell them explicitly that it's right to yell certain things in some circumstances. Good luck.