Wellbutrin actually causes weight loss in some patients. I used to take it and it worked really well for my depression and did help me to lose weight.

I have found the almost all antipsychotics cause some weight gain, I'm not particullarly sure which ones cause the least amount of weight gain. However, you could choose an anit-seizure medication such as lamictal, which is known to help with your diagnosis, and it has a very low risk of weight gain.