What are you measuring with? Did you weigh him regularly while he wasn't eating to make sure he wasn't losing too much too fast? 10 months is a long time but it's not unheard of in ball pythons. If nothing is wrong with his body, usually it's something to do with the husbandry. An assist feed for an adult is an absolute last resort, after all other problems - medical and husbandry - are ruled out.