A few more questions. What kind of hay do you normally feed him? Did you happen to start a new bag or bale just prior to all of this happening? Also, when this first started with the small poops, did you notice that he was having any issues eating, like food dropping from his mouth, not eating as much as usual, drooling, odd chewing motion like raising his head in the air when eating?
I can't say what you should do, but just what I would do in a similar situation. When ever my rabbits have had GI problems, grass hay has always been the best thing to help correct the problem and getting them feeling better again, provided they were eating the hay well. I go by the evidence. You fed critical care, he got worse, not better. That tells me the critical care is not helping but contributing to the problem. I would also suspect that the gas drops you gave may have started the mushy poop problem. Before he was just having smaller than usual poop. What this tells me is that your rabbit has a very sensitive digestive system. All of my rabbits that have had this same problem, were all sensitive to sugars and carbs. As long as I didn't feed those things they did well.
This is what I would do. I would stop the critical care. And free feed grass hay(which ever one you usually feed and that he eats well). Then with veggies it depends on the ones you feed. I prefer to stick with leafy greens. I would definitely not feed carrots, fruit, grains, or any other high carb/sugary foods. Which means it's also important that nobody else does either. Then I would keep a close eye on his eating, drinking, poop, and body condition(making sure he maintains a healthy weight). In my experience, sugary/high carb foods are very often the cause of digestive problems in rabbits. So hay and maybe some of his usual leafy greens is what I would stick with.
