I'd talk to the teacher. The picking at each other, like the older boys were doing, is common among that age group. Most of them pick & tease and are best of buds at the next recess. That's not to say it's OK, but a lot of it is addressed by reminding the boys to quit it. Advise your son to step back and observe & if he's concerned about someone's safety then he needs to go get an adult.
The punch to the gut was too much though. That's not 'fair' among the unwritten code of conduct among boys picking on each other. Hitting a smaller boy is bullying, and I'd ask for it to be addressed by the teacher and/or the principal.
