Well actually if two older boys are punching him in the tummy he is NOT a sap, he is the victim of an assault. If you were hit in the street by two bigger people, would you think it was your fault?
What did the school say? I think they really have a responsibility to sort this out.
I can however understand your worry about yor ds being able to stand up for himself and to maybe work out in advance how to avoid the situation happening in the first place. There are however some situations which can't always be avoided and the important thing is for your ds not feel like it is his fault that this happened but at the same time to be empowered to feel like he has some control and to have some strategies and plans in place so he can try and avoid this happening again. - Not an easy task and difficult even for adults who are victims of assault.
Have you (or he) considered him taking up some sort of martial art? My BIL used to teach this and swears that it really helps young children to have more confidence in themselves. Also maybe he needs to learn more about how to stand up tall and and to answer back. Also when to run etc.
Sorry I can't be more help but I do feel for you - it is just awful when we see our children treated like this.