Bloated stomach in rabbits?

I've been caring for a litter of wild rabbits dropped off at my doorstep and I've been feeding the vet recommended formula for them and watching their intake to make sure I don't overfeed them. They've been started on solids for almost a week now with no issues but I've noticed one of the kits has a bloated stomach. He's still eating and though he's not quite up to par with his siblings he's still active and curious. He's been eliminating fine as far as I can tell but his stomach is so big. I can feel the air bubbles moving around in his stomach similar to having gas. He's so small and young if it required any sort of vet procedure he wouldn't make it. Any suggestions or ideas of what it could be?