Ok, opinions vary to an extent on what to do with an aggressive rat. A couple tricks you can try are putting something you’ve worn into the cage so they will get familiar with your smell, and using baby food to lure them out. Rats love baby food, or baby cereal made with soy milk. They can’t run off with it either. Also, rats are very picky eaters. They try a small portion of something to see if it makes them sick, and only later will they come back and eat more if they decide its ok. So you may just be dealing with that wariness. I would try that, along with patience, and see if he doesn’t calm down. They aren’t generally aggressive animals AT ALL, so I wouldn’t be surprised if that helped.
If it doesn’t, I would force matters. Some people would disagree with me, but I’ve had good luck with it. If you think he really will bite, wear gloves. Also a long sleeved shirt with another shirt over it (like a teeshirt). Get him out of the cage without hurting him (no tail grabbing!) This may take awhile; get rid of anything he can hide in and just get him out. If he freaks, hold him firmly until he chills out a bit. I find having a blanket or towel to wrap them in helps. Once he stops freaking out, put him in between your shirt layers. This allows him to smell you and feel you, but still hide. You might get peed on. Keep him out for at least 30 minutes, but I would aim for 45. rats cannot sustain the fear response longer than that, and he will calm down. Talk to him, say his name, offer him treats (he may not take them). Just touch him and make him aware of your presence for the full time. And do this daily. I would be shocked if you didn’t see massive changes in a couple weeks; usually just a few days is amazing.
See, rats have a complex society, with higher and lower levels. To him, you are a big rat. You need to teach him you are THE big rat, and the benevolent alpha of his rat group. Rats hold each other down all the time for dominance; essentially that’s what you are doing. You are showing him that you are the boss, but that you won’t hurt him. He understands that; its the way they communicate. That’s why I’m not a fan of letting them decide when to be held (at least not with antisocial rats); it sounds nicer, but you need to establish your rat hierarchy! Once he “gets” that you aren’t going to hurt him, but that you’re the boss, he’ll probably act a lot better. They are a lot like dogs in that way.
There is this though- if he’s been alone and not handled for most of his life, and he’s older, he may never come around and be cuddly. It just may be too late. I feel fairly confident you can get him to a point where he isn’t aggressive, but he may never be a “people rat”. So, if you try the steps above and he still just hates being held, I’d let it go.
