Antibiotics kill the good bacteria too. You need to get probiotics and get some down him. Look for oxbow critical care it's the best for ailing herbivores. Feed dandilions if you can find safe untreated ones. They help cleans the system.

If he stops pooping you can try syringing(needless) some oil down him to help lube things up to move. It would be okay to give him some vit c till he's better too but never give it to a healthy rabbit because their livers make it for them and you can mess up their system by giving it to them regularly but for an emergency boost it's okay. A mer half gram of vit C per day is enough or just feed him some high C content foods which is even better than synthetic C. Kale has lots of vitamin C