I have that same exact thing and I am 40. I only found out it had a name last year....never heard of DS before that. I can see just fine and I have never experienced neck pain due to it. I did get picked on in school a little when my eyes crossed, but once I learned how to to compensate by turning my head and avoiding situations where I have to sit next to people to the left of me, people don't really notice. I've had friends for years who never even noticed my eye until I found out about DS and told them...they had no idea!
As far as I know, it can't be fixed, but I am just fine. As he gets older, just teach him how to compensate for the eye and it will be much less noticeable in school. :)