As mentioned above, it might be that your son is set on watching Real but, if not, I'd highly recommend considering one of the other Madrid based teams to watch a match. Atleti's stadium is great and they offer really good (and affordable) hospitality packages too. Otherwise, as mentioned above, watching Vallecano or Leganes will be a really unique and interesting experience. Even buying things like shirts would be far less expensive and a bit more interesting when he gets home. You don't see too many Vallecano or Laganes shirts around.
You could then do a stadium tour of the Bernabeu so your son can experience it.
Madrid is a lovely city! You'll have a great time.