He may be dehydrated. If you can get unflavored pedialyte(used for children), then give him some of that. He probably needs some electrolytes, and needs them
immediately, or get him to a vet right away and get a sub q injection of lactated ringers, which would be the best thing at this point.
By rigid, do you mean hunched up in pain? Do you hear any tooth grinding? If he's not eating and drinking, you need to help him eat. If you have 100% plain canned pumpkin(no added spices) you can syringe feed him some of that, or you can soak his pellets in water, but you may need to blend them in a blender, to get it smooth enough to syringe feed. Feed about 4-6cc per kg of body weight, and do this every 3-4 hours. You can also see if he will eat leafy greens like cilantro or parsley, on his own. You also need to keep syringing water to him, about the same amount as the food. Just be careful to do it slowly, and give him time to swallow, so that he doesn't aspirate any of the liquid. I would stop the oral chloramphenicol, as it may very likely be the cause of him not eating, as that is one of the side effects. If the powder is the trimetoprim, that is ok, but if it is chloramphenicol, then stop using it too. If you are still using the gentamycin cream, stop using that as well. It can also cause problems with rabbits. If you can get a pet probiotic gel like Benebac, or even ones used for horses, then start giving him maybe half a gram of that each day, as he needs it to replace the good bacteria, because of the effects of the antibiotics. The symptoms you are seeing is called GI stasis, when a rabbit stops pooping and eating. So you have to help him eat until he will eat on his own. If his stomach seems tight or firm, and if you hear any gurgling sounds, he may have gas in his stomach, which will also stop him from eating, you may also need to use simethicone(infant gas relief suspension). The dosage is 1cc every hour for 3 hours, then every 3-8 hours as needed. If you have metacam suspension, then that would be helpful as well to help control the pain and inflammation, and get him eating again. You need to make sure to give the right dosage though. The dosage is 0.1-0.2 mg/kg once a day. The medication link I posted, also includes the dosage info for it under analgesics, meloxicam. As long as your rabbit is experiencing pain, he won't eat on his own. So stop the chloramphenicol and gentamycin, and syringe feed him food and water starting right away, and see if he will eat grass hay and cilantro or parsley on his own. Also get some metacam and simethicone. The metacam is extremely important at this point, to get the pain under control. Also if his body seems cold you need to warm him up with a warm pack or warm towels, before you feed him.
Here is some info on GI stasis.
http://www.bio.miami.edu/hare/ileus.html