Disclaimer: I Am Not A Doctor
Recommendations regarding "see another doctor" are spot-on. Given the weight loss I suggest having him evaluated by an allergy specialist, particularly looking for an allergy to gluten. Our daughter (now 12) was having a great deal of difficulty with digestive issues and stopped growing at age 3. Once we found she was allergic to gluten (part of wheat, barley, and rye) we eliminated it from her diet and she's been fine since. This may not be the problem with your son but since it's often overlooked it might be good have the blood test performed. In our case the blood test indicated that she had the allergy; the doctor then wanted to do an intestinal biopsy, which required general anesthesia, which we declined to do - we didn't feel that it was appropriate to subject a four-year-old to general anesthesia, and since eliminating gluten from her diet produced immediate improvements in behavior, food intake, and growth we decided the more invasive procedure was not needed.
(And yes, I'm aware that in recent years gluten has gotten a bad name, and eliminating gluten from the diet has become something of a fad. For those who are not allergic getting rid of gluten will do no harm but probably won't do any good. However, for those who are allergic to it, such as my daughter, eliminating gluten can only be beneficial).