Give how bloated he looks in that one picture (which might be misleading - pictures are sometimes unclear), I think his only hope (and it is a slim one) is to take him to a vet.
I think it best if you take him out of the tank and keep him in a container with fresh, dechlorinated water. Change the water twice a day, and make sure the container is safe from pets and children, etc, and that he can't get out (covered or deep). This is to keep him from being bothered by his tank mate and to protect his tank mate if it is contagious.
I don't know how to treat bloat. If you search this forum for "bloat", you may be able to find some recommendations by some of the reputable keepers. Look in the oldest posts - the veterinarians used to be more active in this forum. The vets will sometimes have "HerpVet" under their names.