Flat-Coated Retrievers

We have a 6 month old flat coated retreiver -we've had him for a week now. The breeder is reputable and has assured us that the pup was dewormed. Not so He is infested and we discovered he is now vomiting up roundworms. We are afraid that he has had this since birth and that it may have inhibited his development. He seems smart but is skiddish, hard time feeding him although we are presenting the raw diet he was raised on. Does anyone have experience with roundworms and development problems?. He vomits in the car too and I think it is because he is sick. He had never ridden in a car until a week ago. My husband wants us to return him to the breeder but of course he is so sweet and beloved by many already. Opinions on returning the dog vs. trying to work out the problems? The last straw for my husband is that the dog tries to eat his own feces all the time . Can this habit be broken? Appreciate any help I can get.