I think some people deliverately miss the point just to give somebody a hard time. Yes in the beginning, you did overreact, but it is very strange he has gone to such great lengths to keep following her.
Even if you were being a bit weird about her at first, a loving partner with nothing to hide would be like 'ok ok, I'll stop liking her pictures' and just stop it. How hard is it really? There wouldn't even be anyway of you knowing if he still likes them in his head when he scrolled past them. He just didn't have to press the button.
So for him to keep re-following her from different accounts makes it seems like he is trying to get her attention. If that is, they're not already in contact.
As Loulzze said, just talk about it without getting wound up. Explain that in the beginning, you might have overreacted but his ongoing actions have contributed to this.
I don't think I would break up over this but in your position now, I would want assurances there is no communications between them on private messaging etc and to hear why he felt he had to go to such great lengths to keep 'liking' her photos.
I get OP that somebody can most of the time be rational and sane but sometimes a little thing can trigger your spidey senses and really get in your head. I think most partners would understand that.
If your partner is suddenly liking pictures of a girl posing, it's clear he's attracted to her. That's never going to feel great no matter how cool people say you should be about it.