Definitely go in and chat. Schools usually have a range of strategies, and I suppose you need to be his advocate to make sure the ball is set rolling to implement those.
I'd be inclined to act sooner rather than later, ie. your dh going in on Monday, but that means, I suppose, you not going (twice in a week may well be over-kill). Though I think you are almost certainly going to have to go in again, later in term - it's rare that it all gets sorted very quickly, so another trip may well be on the cards. Perhaps make sure you both go on that one?
I've found it's helpful to both go, somehow it seems so much easier if the pair of you are there to fill bits in, and so on.
I'd expect the school to do some "circle-time" stuff, at this stage. General talk about what sort of behaviour is expected from everyone at school, and in the playground, but done in a subtle, child-friendly way.
Poor you, and poor ds. Glad he likes school.
You're right about not telling him to hit back. That really won't help, not least because then the school have to discipline him, which will just make everything very unclear.
Also, avoidance, if at all possible, is a great strategy. Are there any other children he can play with? At the very least, he will be less available to target if he is with others. Can you facilitate and strengthen those relationships out of school, or are you both working, and not able to do playdates, and so on?
Good luck; it is awful when things like this happen to our children.