Well, here is what I know, although I am not a DR, obviously
The AP medications like Risperdal, Seroquel, Zyprexa, etc I believe have an affect on the pancreas and insulin, i.e. lowering blood sugar, which makes us really, really hungry. It is a biological/systemic side effect of these medications.
It also increases cholesterol sky high, which must have again to do with the pancreas? Matt, at age 14 had a cholesterol count of 500. Now granted, some of this is because he ate everything in sight, but there is also tons of medical evidence that these medications do increase appetite and cholesterol. In particular, Zyprexa.
Regardless, it was and is the only medication that really helps him think straight. He recently went off of it due to the weight thing again - it is hard to manage an appetite of a lion when you are merely a mouse - but I wish he would have stayed on it. He thought so much more clearly.