As one who is searching for the cause of all my medical woes, I've come across gluten intolerance/celiac disease.
Once a person really starts researching this, it is absolutely unbelievable how many chronic and autoimmune diseases are affected by and/or caused by this condition. Smelly/obnoxious gas, diarrhea OR constipation, gastointestional reflux (also known as GERD), fatty stools, abdominal pain and/or bloating/abdominal distention, failure to thrive, picky eater/not wanting to eat -- all of these are signs in children that point toward a possibility of this disorder.
Smelly gas from a child is not normal; in fact, it's not normal for anyone. As a few others pointed out, the cause is rooted in something he is eating that's not agreeing with his digestive system. It's been my experience over the past year (I'm 41 and still trying to find out if I have celiac; I do have gluten intolerance) that you're better off going to see an alternative doctor (Integration doctors are great!) than a regular conventional doctor, because a regular doctor in all likelihood won't know anything about celiac disease/gluten intolerance and, most probably, won't test for it. That, at least, has been my experience with both myself and with my 8-year-old daughter, who has had problems since birth and with whom we have been offered no help from her pediatricians.
I hope this info helps, and I wish you and your son good luck.