This is one of the new arrivals right? If so, I would get ready for a visit from Amyloo or Brook. Better safe they sorry and you'll likely see accelerated breathing tomorrow if it is the case.
If he is still eating, keep feeding him small/tiny portions frequently. If he breaks down, a little weight on him now is going to be helpful when he skips a few meals.
Hopefully it turns out to be something mild.
Good luck and post and update tomorrow.