Motillium is completely safe for long term use. It is actually a drug used for stomache complaints but has a nice side effect of causing lactation (which is why we take it). People stay on motillium for years and years, so don't panic.

Your Dr is a jerk- you can breastfed for along as you like. If your Dr won't prescribe you motillium contact your local lactation nurse and hopefully she can guide you to a Doctor who will.
Good luck