What to do, my bunny is being sick?

I thought my 9 month old adopted rabbit had a delicate tummy, and was noticing tiny wet brown poo's in his hutch, not somewhere he would normally go to the toilet. I had him castrated about a month ago. When I think his tummy is bad, I feed him lots of hay and not too much fresh veg (as advised previously) but I've realised that these tiny little wet pellets aren't from his bottom but come out of his mouth! It's almost like my poor little bunny is being sick. How can I make him better?