I got this from a website. I hope it helps.
Male rats are very reluctant to accept newcomers, with one exception. If you introduce a 5-8 week old rat to an older male, the older one will adopt the younger as his son and will be very gentle with him. Later, after puberty hits, it is much harder for male rats to accept others. Even males that do get along will occasionally become so territorial at puberty that they will beat up their former friends. Males usually always fight to some extent and do a lot of boxing, power grooming and flipping of subordinate rats. Male rats seem to enjoy playing rough, but if one is getting picked on too much or is getting hurt or scared, it's better to separate them.