I have chins in colonies who have babies all at the same time and nurse everybody else's kits and their own. Right now though I doubt she has milk enough to sustain him. You could try rotating him and the other mom's kits in and out once they are born. The problem here is the girl beating up the boy. That same girl would most likely turn on any new kits your other female has.
What about dad? Most dads are awesome with their offspring and he would hopefully at least keep the little guy warm. Monitor it though. Since he's having issues, he may be rejected by any other chin.
Personally, I would separate the two that you are hand feeding now. If she is beating him up, it could turn ugly very quickly. It takes very little time for her to cause him to be injured so badly you will lose him. You should have a heating pad under their cage at all times, but only half the cage, even if you think it's warm out. If he is too warm, he will move away from the heat to the cooler part of the cage. If he isn't warm enough, he isn't going to eat well.
It's hard to give advice in a situation like this. I know what I would do and I know what works for me, but unless I say after every sentence "be careful, watch for this, be sure of that" and discuss every possible outcome, then a kit could end up getting hurt.