Are there anti-psychotics that are proven not to cause weight gain?

No: No, different people respond differently to antipsychotics. It is fairly typical for children to have marked weight gains (especially with the first antipsychotic that they use). Geodone is pretty weight neutral. Seroquel (quetiapine) appears to do fairly well in this manner. However, even if most people don't gain weight on a particular medication, it doesn't mean that you won't. No guarantees.