Rat agression/depression

Question
I got my ratty, Roger (male), almost 2 years ago, making him about a year and 11 months old. He bonded with me from the time I got him, and since he didn't have another rat that he lived with, I was his closest companion.
When he was about 7 months old, my boyfriend got a male rat (probably about 6 weeks old) and we tried to introduce the boys. It went over poorly, Roger ended up attacking the baby pretty bad, ripping out a lot of fur and bit him, drawing blood. After that, we kind of gave up, and Roger stayed with me, and the other rat lived in a separate home with my boyfriend.
But basically ever since then I've seen numerous behavioral changes with Roger. Shortly after being introduced to the younger rat, he would do strange markings on me and other parts of the room, where he basically drags his butt/testicals on the surface of wherever he is walking. It bothered me when he would do it to me, because he no longer seemed comfortable with me...he was always tense and acting like he was going to be attacked. After a while it got to the point where if I placed my hand by or over him, he would immediately attack my hand and give me a good hard nip.
Now, several months later, he is still very...bi polar. He'll run around the room like a spaz, and then come by me, mark me, and then randomly start nipping 'til it gets so bad that I'm actually afraid of him and have to somehow scoop him up and get him in his cage.
I feel horrible and guilty because it's too the point where I don't even take him out and interact because I'm afraid he will hurt me. I don't know how to get back my sweet rat. He's 2 years old and I'll only have him for another year or so, and I want him to be happy for the remainder of his life. I know he's depressed now and unfamiliar with me due to my neglect.
I'm just not sure how to "start things over".

Any advice you have to give would be much appreciated.

-Mary

Answer
 What likely happened is that Roger always felt like he was the only male in his 'territory' and now that he knows another male exists somewhere out there he is growing territorial.  A lot of it is due to hormones which can be triggered into overdrive by such behaviour.  He may also be able to smell the other rat when your boyfriend comes over.  The dragging his testicles thing is him marking his territory, he must feel threatened and feel that he has to set up boundaries so no other rats come in.  This is common with rats who have lived alone for most of their lives, they get used to it and don't want a companion and usually brutally attack any other rat (except a female, usually) that comes near them.

 There are two options that may help.  One, make sure your boyfriend washes his hands before he comes over to reduce the likelihood that Roger will smell another rat.  Second, you may want to consider having him neutered.  Even though he's getting up there in age if he is in good health it is a fairly simple surgery that should give him no complications.  It often helps cut down on hormonal aggression.  If neither of those things help, there isn't a lot you can do besides just continuing to handle him gently and hope he gets better.  Some rats display what is called genetic aggression where they are basically wired to turn aggressive to other rats around 6-8 months, but since your rat lived alone he may not have had that triggered until he was introduced to the other rat.

 I certainly do hope things work out!  Good luck!