According to my vet the number one food ALL rabbits should eat is timothy hay. It's high fiber will also harden up those runny poops. However since you have a baby you can use alfalfa based hay until the bunny is about 1 yr old.

It sounds like Thelmer has gut stasis, which can kill a bunny if left untreated. Let me join the previous advice and be sure you get a bunny specialist vet. Thelmer may need a dose of Reglen, which the vet can also give you some extra to take home in case it takes a couple of doses to clean things up.

Good luck!