I would find out what he means by "being mean".
Because if they're saying "you can't play" or similar, then it's worth speaking to the teacher about. IF that's happeneing more than occasionally.
If "being mean" means that he comes in and says "let's play tag" and they say "well we want to continue with what we're doing" then that's not mean, and you can discuss with him how to join in.
Or it could be he walks up and says "I'm brilliant at football" (which a lot of boys do) then he's liable to get the response "well, I'm/Sam/someone else is better than you".
Also is he always approaching the same group? Not saying that they should tell him he can't play, but it can become a habit if he's always asking the same people.
And I wouldn't dismiss the girls as possible friends either. My ds is very typical boy, but in year 3 and his friendship group (which has changed each year as they're mixed up ) has always included at least one girl, and often he's been best friends with a girl-and sometimes they've been real girlie-girls too.
So I would at some point when he says they've been mean find out how the conversation went, and who with. Bearing in mind that he, as everyone else will, will give a self bias report.
I would then use that to help him talk through a better way of approaching people and making friends.
But also I do agree with not being too anxious about it. I didn't have a best friend until I was at secondary. Did it bother me? Not most of the time. The times it did bother me was school trips and things with partners as I tended to end up with who was spare, which I found a bit stressful waiting to see.
I have noticed over the years (my oldest is year 9) that the parents who grret their dc at infant level with "who did you play with?" type questions and are obviously anxious about it, often produce children who are anxious about making friends, which can mean they approach others either over anxiously or sometimes quite demandingly.