I would keep him in with the other litter and just see how he does. So long as he's gotten a bit of colostrum, he can wait until his new momma goes to feed again.
He looks a bit runty, in my opinion. It could be that her previous owners (did you get her from a rabbitry, or a private person?) didn't give her quite the nutrition she needed for best kit development. Or, it being her first litter, it could just be a poorly kit and her next litters may be better.
If he continues to do poor, I'd quietly put him to sleep, personally. I generally give my kits a week to two weeks (if they're really hanging in there). If they don't pick up or show improvement, I put them down with a small quantity of dry ice. Some breeders rather nature take it's course, but I feel it's more stressful to the doe to have a struggling kit slowly die off, then to remove it and put it to sleep.
Good luck with the little guy, I hope he makes it!