Too bad he didn't fight! My kids are all learning that anytime someone strikes them or lays a hand on them, to fight back. Simply because bullies only pick on the weak. Once there has been an altercation, no matter what the outcome, the bully moves on. I would pull out all the stops.
1) I would rehearse verbal comebacks (though they won't have much effect since he's been overpowered physically in the past). But definitely come up with come backs for teasing about liking the girl. Even if it's just "Oooh, Good One, I never heard THAT before, You're so FUNNY" or something. And have your son give the girl a "sorry I was mean because of those jerks, you're cool" type apology if he can bring himself to.
2) Encourage him to fight back physically. Unfortunately "ignore it" is deadly advice these days. Anything ignored gets worse. It's an old fashioned "turn the other cheek" idea which was popular in the days when people went to church, and schools switched bullies (sorry, true) and expelled them. Now that schools will barely address it, the kids have to fight for themselves. And even in the old days, kids fought for themselves. My mom (Iowa farm girl now in her late 60's) threw a snake at some boys who were throwing rocks at her sister. Since she was the only girl who would ever pick up a snake, no one ever messed with her again. (this was off school grounds on a long walk home-the only place bullies dared to strike since they were scared sh_tl_ss of the teachers and principal).
3) I would file much more serious complaints with the entire school staff. He's still young enough to be on the cusp of your intervention.
4) I would send copies of the complaints to the bully's parents along with a note, or call them up, or knock on their door. I'd tell them your son has been instructed to fight back, and you hate to not "just let boys be boys because this is no big deal, but your son is starting to hate recess, and you wanted them to be aware of what is happening." Offer to have a family get together pizza night and include the boys or something.
Explain to your son that if you don't report it "for fear" that the teasing will get worse, the bullies are winning. Let him know this will pass if everyone confronts it. But if it's left alone, it will only get worse. It's gotten this bad because no one nipped it.
Good luck, this is so awful, I feel for you!