Check your cage temps if they are too cool, that could be why he's not eating. Also, if his tail is looking thin, then he is deffinitely loosing weight. Getting a fresh stool sample to a vet for a fecal to check for parasites would be a good ideal. Also, if you are always feeding the same kind of insects, he may be bored with them. Try some other kinds of insects such as silkworms, crickets (if you are feeding just mealworms, or try mealworms or superworms if feeding only crickets) or butterworms. Also, he may not be eating as much as he is older, and reached his full growth, meaning he doesn't need as much insects to grow as would young leopards.