My dog is extremely thin he is about 10 months old... we have changed his food brands countless times but after every meal he immediately poos... he even eats his poo as well... it is a pitbull.I have taken him countless times to the vet ,the deworming isn't helping either

If his hair is dull and brittle as well you have described all the normal symptoms of tapeworms. The Doctor should have caught that if that's what he has. How long ago did the Dr. de-worm him?

Sounds like it's time to go to a new vet.

Has bloodwork been done?