Have you looked at what he eats? Sometimes nutrition has a part in it. As his siblings are OK, perhaps it is something that he alone does, such as too many soft drinks, too much of one particular food. Strangely enough, with some people it is wheat - they don't specifically have celiac disease, just a general allergy to wheat. Also, wheat can trigger other allergies. It might be worth trying cutting out one particular food or drink for a couple of weeks at a time, just to see.
This is just a thought.
Meanwhile I would expect the same standards of behaviour from him as from the others. I know it would be harder to enforce, but I can't see how giving too much leeway would help. As long as he knows you love him, you can enforce the boundaries.