I AGREE to stay away from commercial drugs/laxatives. MOST are GARBAGE! My doctor had me take docolax at one point, and it nearly killed me. He didn't even tell me what it was for. I started recovering the day I went off of it, and 3 days later was back to normal. You should do two things:
1. Increase fiber through things like oatmeal, bran, etc... MAYBE psyllium ALSO, Omega 3s could help in several ways.
2. #1 will eventually probably help the smell, but a good part of that may be because of what he is eating. MOST of the crunchy foods, and spices are probably fine. As I recall, brussell sprouts are HORRIBLE culprits.
As for something that could help BOTH, be tasty, AND give him vitamins, etc... Have you considered a nice plain green salad? Cucumber, carrots, lettuce, tomatoes, perhaps cheese, and his favorite dressing. Personally, I like french! Don't go too far with vegetables, etc... Beans, for example, aren't always digested right, so THEY can cause problems.
Carrot sticks and cheese and nuts can make nice snacks also. AND, keep in mind that peanuts are NOT nuts! They are STILL great BUT, if your son is alergic to peanuts, there is a GREAT chance he ISN'T alergic to REAL nuts like walnuts, almonds, etc...
Smoothies ARE good also. But be careful about some vegetables that can make the smell worse.