Some issues with the new boy

[eveliens]

I recently (a few weeks ago) adopted three boys. They supposidely lived together before and had lived together at the shelter, but once we got home, nope. Two of the boys are very sweet and friendly, get along just fine with other rats, and generally have no problems (other than they're grabby with food). However, the other boy has some issues.

At first I thought he was just still overly aggressive (recently neutered a few weeks ago), so I tried to introduce him to my youngest, most submissive female rat... HA, she kicked HIM around at 1/4 his size! So it isn't that he's overly aggressive, it's the opposite. He's a doormat. But he doesn't seem to realize when he's starting to bug the other rat and won't stop, so they pummel him, then he cowers. So every time I saw the end of a scuffle, it looked like he was fighting because the other rat cornered him.

Another bad habit he has is that he'll chomp for no reason. The first time he sunk his teeth down to the bone on my finger. Since then it has been hard pinching, but it still hurts. I don't smell like other rats or food. He always does this when he's out playing, so it's not cage related. He seems like he wants to come see me and will watch me in the cage and likes to sniff my face (thankfully hasn't bitten it yet!).

It almost seems like he just never learned any social skills towards other rats or towards people. Any tips for this boy? He actually seems quite content to be alone (weird) and he's at least 1 year old, but I think he's older by the way he walks/looks.