There could be many reasons why your dog is loosing so much weight. Take him to the vet. Your dog could be suffering from tapeworms.
What dog food is it on, how many meals a day does it get, how much is in a meal, eg weight, I have a 2yo boy who gets about 2 meals that are 1.25kg ea and he is at a healthy lean weight, but if yo uare feeding low quality food the dog will not be able to digest it properly and get no protein from it therfor will not gain weight
I don't think my dog has worms but is still very skinny