I am not up to speed with what the medical form asks for, but I would have thought this situation would have very little relevance. You say there was some treatment for a childhood condition that no longer exists. I would have thought a very high number of children are treated for something or other during the course of their early lives and is it the end result that matters.
If the question asks for details of any treatment it would be appropriate to list any, with a qualifying statement. If not fine. The medical is to asses the health of the candidate now, and their likely health in the foreseeable future. The sort of problems that may have a significant relevance for an adult may be less significant in the context of a childs development.
I would reassure him and contact the medical section of the CAA if you require qualified guidance, or perhaps in the first instance contact a local aviation medical examiner in your area.