How do I get my 1 yr and 3 month Boxer to gain weight?

I have had my dog since we got him from the breader. He has always be very thin. Vet told me to watch him after his last set of shots in March 2009, but said I should see 2 ribs with a Boxer. In Oct. he got really thin. I could see his back bone, 4 ribs and his hips and it seemed to happen over night. Took him to the vet. He had a parasite (sp) and put on meds. Rechecked his stool and seemed to see round worm, but they were not sure and still the parasite. So, another round of meds. Started to look a lot better. We moved two weeks ago. Weight feel off really really fast. Took him back to the vet. Stool was good, but now he weighed 10 lbs less than he did the first vet vist. (62lbs in Oct. 56.6 now) They sent his stool off to a lab. Vet called me a few days ago and said nothing was found, but started him on anibiotics in case it was a bug. I want to cry everytime I look at him. Vet is going to take blood if he does not gain ANY weight or loses more weight within the next two week. This dog is my 6 yr old son’s best friend. Please let me know if you think there may be something else going on with him that we may be missing. I feed him twice a day, 4-5 cups (total). Vet recommended I take him off Iams puppy and put him on this Iams Active dog food. I have to wrap the pills in cheese to get him to eat them, and even then he eats the cheese and spits the pill out on the floor and walks off. I think I am going to go by the puppy chow you recomended in the other posts tomorrow. We have a border collie that is overweight, so this is new to me and it is breaking my heart. Thank you.