I really feel for you. It must be devastating to see your son going through this. I have a 10-yr-old son and he has gone through a situation like this in the past, too. In our case, I talked to his teacher at his school and asked her to keep an eye on them and tell me if she witnessed something. (I got the OK from my son before I spoke with her.) This actually alleviated the situation a bit, so the teacher must've had a talk with them or something.
I also talked to my son and tried to find out if anything specific happened that led to the situation so that I could come up with what he and I should do. Fortunately my son tells me pretty much everything that happens at school, both good and bad, even though sometimes it takes a while for him to open up and tell me the story if it's a bad one, but him telling me the story helped me a lot since I didn't have to stress over something that I had no idea about.
In my opinion (and I'm sure not everybody will agree with me), I think it is totally fine for your son to argue with his friends once or twice in situations like this, because that will let them know how badly he's hurting inside being treated like this and also that he's not just a coward. I feel that, with boys around this age, if you don't stand up for yourself at least once just like your son did, it's going to be taken as a weakness. But frequently going back to them and having arguments with them will probably only worsen the situation just like you said, making him look like a pushover.
Looking at kids at my son's school, especially boys around his age, I'm assuming this sort of thing happens randomly to pretty much any kids in a group of friends, and not just to specific kids. I know it sounds bad, but it seems like it takes turns for no specific rearsons... I think if there isn't anything specific that caused this, after a while, your son's friends will probably get bored of or forget about what is going on right now and everything will go back to normal like nothing happened one day.
He had an argument once with them already, so it might be best for your son to leave it at that for now, and hang out by himself or with other friends who welcome him and respect him. I've told my son in the past that he absolutely doesn't have to hang out with people that don't want to hang out with him, and he certainly shouldn't have to ask or beg anybody to hang out with him unless he's done something bad to them. Do you know any of their parents well enough to be able to talk to them about this? If you do, talking to them about what he's going through might be able to relieve your stress a little.
I am very sorry this is so lengthy and all scattered. I'm not an expert or anything, but as a mom who has a boy around the same age, I hope everything will work out the best for you and your son in the end.