You don't have a rooster, you have an immature cockerel. Your girls are immature pullets too, not mature hens. The behaviors of juveniles can be quite different from the behaviors of mature chickens. I think that's part of what you are seeing. And two of your pullets are a month older than the cockerel. They are probably the ones doing the damage.
Anything is possible but that's unlikely. Sometimes older, more mature chickens will beat up less mature chickens. Sometimes older girls like to beat up on younger boys. Size generally doesn't have anything to do with it, it's more maturity level and sex.
I assume they have been together for several weeks if not months and this is the first problems you have seen. He is getting close to 4 months old. My guess is that his hormones are hitting and he is showing interest in mating the girls. The two older girls probably told him "No".
I'm not sure what your facilities look like or how big they are. Usually the more room the better. More room gives you more flexibility in what you can do and gives them more room to separate if there is conflict, let alone room to avoid each other to start with.
I agree, the damage doesn't look bad. You have that second coop if you need it in an emergency. I'd wait and see how it goes. Sometimes it actually isn't that bad as they go through puberty on the way to adulthood. Sometimes it gets really rough. You may change your mind about separating him from the flock.