My daughter is 10 months old. She just got over this. She lost a pound in just 3 days. If you are worried, ask your doctor for a prescription for Levsin (the generic form anyways). My daughter's only other symptoms were being clingy and throwing up. She ate fine. She played fine. The bananas may be one of the culprits, though, since they make you go. See if the Levsin helps, and if not, demand that your doctor do some uninvasive tests (a fecal culture or something). We let our daughter eat as usual, but we gave her bland foods (this was more for our sake because her diapers were already more than we could handle), and she nursed. My husband also ended up with the same illness. let's just say that between them, I was CONSTANTLY airing out the house.
I hope that this helps you some. The medicine won't hurt your son if given in the right dose. They will give him liquid. It says to mix it with food or water. it didn't work until I started giving it to her straight. If this doesn't clear up the problem, remember to demand that your doctor look into it more closely. Losing that much weight is not good.