I'm so sorry to hear about your sick bunny love. I went through what you're going through just over the end of year holidays. My Bella stopped eating because of dental issues caused by some underlying, probably liver related disease. Unfortunately, I lost her in mid-January because we weren't quick enough at getting her diagnosed. I think you have some options treatment wise, and I'm sure others on the forum can offer better advice on that than I can.
However, I definitely went through the not-eating-losing-weight issue with her, and I probably owe my last two weeks with her to being able to keep her weight up. If he's having trouble eating, my best advice is to double down on the amount of critical care. As far as I know, there is no other magic weight gain bunny rememedy. Critical Care is what Bella was always put on at multiple vets. That, and if hospitalized, an IV drip to get fluids up. So, if you're at home, I think your best bet is Critical Care. I know you're saying that he's not eating it, but how are you going about feeding it to him? There are numerous ways of trying to convince him to eat it.
I tried a few different ways that worked for me. The first was to blend/mash up some of Bella's favorite treats (carrots/banana/strawberry) and mix it with the dry critical care. It developed into a paste and then I rolled it into pea size balls that she could work on over night. It was convenient to me to count the number of balls before I gave them to her so I could track her progress and get an idea how much she was eating. I also mixed the critical care into a drizzly consistency using a half-and-half mixture of fresh apple juice and water, instead of just all water. This she was able to lick up on her own from a plate. Once she started to lose enough weight, however, I started syringe feeding her. Luckily, she LOVED getting nutrients through the syringe, and would like, scale her cage trying to get to it when she saw me coming. So, it wasn't too painful for me to give her large quantities of CC at once. If he won't take it any other way, give it to him through syringe. I have had to force feed her through the syringe in the past, when she was feeling really really sick, and it's just a necessary evil. Try to get it into him as many different ways as possible!
Good luck, I'll send positive thoughts your way!