How about taking him to the vet and having a complete work-up including Feline leukemia and aids test, Bartonella test, CBC, Chem and T4 test, check for giardia etc,etc. I am sorry to say that there is no easy way to diagnose and treat weight loss of unknown origin. It could be caused by a congential abnormality, latent parasites, endocrine disorders (such as diabetes or hyperthyroid), anemia from a variety of causes including infectious causes, cancers, and more. Although your cat has been treated for parasites there are so many parasites out there that it is virtually impossible to treat for all of them with one or two antiparasite treatments especially if infested with an unusual parasite. Treatment could be easily accomplished once the proper diagnosis is made. For example, treatment could be as easy as giving him a daily dose of prednisone.
No we took him to my mother-in-law's house. She feeds him twice a day and he has accses to water all the time. He just got worse when we took him there
You moved, and left your cat behind? How about trying to find him a new home?