Looks like a popped abscess.! If you can smell it does it have a bad smell? I think it's healing on its own not too long ago I had a rattie with a popped abscess so we just cleaned it a bit *kinda like a saline flush* and it was closed and perfectly fine within a few days.
Are they neutered.? If not I would recommend doing so if they are fighting. Do they draw blood on each other or is it just play.? All my rats are young so they are still learning. If you can flush the wound a bit that may help it heal faster and if he starts to seem off or the hole doesn't look better soon I would take him to the vet.! My first abscess encounter I got her on antibiotics but the second we didn't give any antibiotics as it was already healing so we kept it clean and now you can't even tell.!
Where is the rat's penis in relation to the wound? I couldn't see the genitals and I wanted to make sure it wasn't actually his penis that was infected.
I usually am concerned when I see wounds on my rats' stomachs. Back, neck, and shoulders are usually where I see injuries like this. I very seldom see wounds on their tummies and that is almost always caused by another rat bullying the injured rat.
How long have the rats known each other? If it's been a while, I would suggest possibly removing the aggressor for a neuter. Male rats love to establish dominance, so their posturing and rough playing could be mistaken for aggression and fighting. If there is a lot of shrill, stressed squeaking and squealing, I would remove the aggressive rat.