Man, you're not feeding him enough. A single or 2 mice a month can't get him grow healthy . Ball pythons need to fed at least once a week. Aside from that, proper enclosure and the right setting of temperature and humidity is a must. Don't wait for that 6-month death toll to come before you notice that he's not gaining at all. Whatever reason you're not feeding him right, I'm sorry for that but you need to hustle out more effort for him.