Antibiotics are pretty hard on the digestive system as they're designed to kill bacteria - both good & bad. It's pretty common for their poop to get green & weird while on antibiotics because of that. If you're concerned, it wouldn't hurt to send a picture to the vet, just to double check with them. But if he doesn't have long left on his antibiotics, you could wait & see if it clears up once he's done with the meds. It'd also be a very good idea to add probiotics to his food, which can help re-establish good bacteria in the intestines. You can either get small mammal Bene-Bac from pet stores or from your vet (or online), or you can get acidophilus, which is found in regular human pharmacies, near the vitamins. Just make sure you don't give it at the same time or right before the antibiotics, or the meds will render it useless.