V., I totally agree with SM's approach. As she suggested, you can talk (firmly but still respectfully) to the driver once more and invoke the clear idea that you now want to speak to his or her supervisor -- often that is enough to get someone to take action. But at the same time, it is important to teach your son, today, to stand up for himself on the bus tomorrow morning. I don't mean hit or pinch back - no way, and your son likely will be the one who gets in trouble if he does that, not the kid who's the instigator. If your son verbally and strongly defends himself even once, it may cause the other kid to stop. The child may not be malicious; he's probably pushing your son's buttons just to get a response out your boy after a long day at schol, and a strong verbal response for the whole bus to hear may startle the other child out of it, especially if the two boys otherwise get along. I would work on this with your son right away, before another day passes, and role-play it, so he gets used to the sound of his own voice saying, "Stop that, you may NOT pinch me!" Not whining, not pleading, but a clear declaration that his space is HIS space.
I'd give it a couple of days of your son defending himself and if that does not make the child stop, or your son is just too shy about doing it (which is fine! He's only five and this is all new to him, and he may fear upsetting a "friend") -- then immediately request that your son have a new assigned seat mate on THAT DAY's bus home and afterward. I don't know who at the school would handle that request but I'd go in there in person, not call on the phone, and calmly tell them his seatmate needs to be moved that afternoon. They may react quickly and well, or they may be chilly towards you -- sometimes school administrators see such requests as interference by "helicopter parents" hovering over their kids too much--but if your son continues to be handled by this child, you have to intervene.
Remember one thing amid the natural upset you feel: The other little boy may indeed be a seasoned bully, but he may also be a kid who just doesn't have much maturity yet, or who has older siblings who "play" with him this way, or parents who don't set any limits, etc., so rather than make him the villain when you deal with the driver or school officials, you could approach this as "This boy seems to have a hard time keeping his hands to himself so they need to be separated for now."
But do look at helping your son develop skills to defend himself and speak up. No one should touch him in any way he doesn't want--now or in the future, so this is a good potential opportunity for him to learn.
When I was a kid I got bullied on the playground by some big girls in my class but it didn't last long because my mother took me aside and told me to defend myself in no uncertain terms. That gave me the guts to speak up and things didn't come to blows because the bullies were so shocked that the "nice girl" told them to back off that they did indeed leave me alone after that.