You need to either buy Critical Care (which is an herbivore feeding supplement designed for rabbits, guinea pigs, and degus), or take his normal pellets, make a mush with them, and syringe feed him that.
We are as gods to the beasts of the fields. We order the time o' their birth and the time o' their death. Between times, we ha' a duty. - Terry Pratchett.
"Men have forgotten this truth", said the fox, "But you must not forget it. You become responsible, forever, for what you have tamed. - Antoine de Saint-Exupéry