Drinking is *really* important for anyone taking medications (kidneys need to break down and eliminate medications through urination). I don't know how old your child is but if a toddler or older, check with your physician about using "pedilyte" flavored ices for children. I think they are available over the counter in the pharmacy and I would think would be available "on line" in more flavors and possibly cheaper by the case. Sometimes it's easier to get fluids in kids with these (for any reason <G>).
I have no idea, medically, what's going on with your son. Asking for details from your physician or a nurse clinician working with your physician is essential. I *do* know that even though I wasn't maintaining a therapeutic level of Tegretol in my blood stream, I became toxix on the metabolytes (building blocks of tegretol that the tegretol was initially broken down into in the first step of my body using and eliminating the med). It did cause many of the symptoms you describe.
The reason I'm mentioning it is because when the tegretol was discontinued, these symptoms subsided as my body broke down the remaining medication in my system, over the following days. It will pass...
I had to stay hydrated and avoid medicine that "took priority" in my body's hierarchy of breaking down meds. For example, I became toxic on tegretol when given Darvocet, because in my body, until the Darvocet was eliminated, the tegretol kept circulating and building up with each dose. There are very specific drugs that interfere with blood levels of anticonvulsants and elimination.
I'm not saying this is what's going on, but do know that if the discontinued medicine is causing your son's condition it will steadily improved over the next few days.
Really try and get your son to drink. Any creative way to get fluid into him is essential
ice cream is good... adding even a little water to cereal, italian ices, any way you can. It is amazing how much better the body does what it needs to do with proper hydration.
Don't know if any of that helps and always check with your doctor before doing anything