I'm sorry you're going through this. I've had american mink and ferret bites, cat bites, a very small number of real parrot bites too... and i've found nothing is as scary as a bad rat bite! they remove chunks with their sharp teeth and are so fast that they're difficult to avoid. Super scary!
I had 7 rats in the past, 6 females (2 groups of 3) and 1 male. My first 3 females were neutered at a young age to try and avoid them from getting certain high risk, frequent types of tumors as they age... so I decided to go for a male rat and add him to their group. I liked the bigger size and he came from a friend of mine who breeds for temperament. (hormonal agression in males often having a genetic cause.) You're supposed to wait until a male hits one year old before you breed him to be certain that he's not agressive and won't pass that trait down to his male descendents and she never made an exception to that rule. (Females are not concerned by hormonal agression, they very seldomly bite.)
Well... long story short, I lived what you're going through and it was horrible. He was great with his female friends and until he hit 10 months, he was great with humans too! and suddenly completely changed. He was clearly hormone agressive and unlike parrots that you can work with, rat hormones don't ever go down. That kind of agression won't go away. You can still train them with positive reinforcement of course and take precautions etc. to avoid bites but you have very little chance of getting your sweet pet back if you don't neuter him.
In my case, his breeder ended up taking him in because I just became too afraid of him to properly work with him and his territorial cage agression displays towards me were stressing out his girlfriends. He really calmed down over time with proper training but he was in expert hands. I wish you luck with yours!