Truly Crissie, there are lots of reasons for the fluid build-up. Most likely it’s related to the liver not working as well as it should. Are his liver numbers like AST, ALT and Alkaline phosphatase up? If that is the case then the fluid is ascites which happens when the blood from the body cannot pass through the liver as easily as it should, then some of the clear parts of the blood leaks out and causes the bloating. If it becomes uncomfortable for him they can drain it off called a paracentesis and if he gets it fast and often they can put a permanent drain in then it can be drained at home.
I would ask the doctor because it can also have other reasons and they may just be able to give him a diruetic (water pill) to make him urinate more of the fluid out.
I would definitely have him take his weight regularly to see how fast he starts gaining any fluids.
KrisV