Does your son contract his services out because he is so busy and he provides good quality.
His is a pretty specialist field, so many businesses won't have someone who can do the work as quickly, with the know-how to reach the high standard. Whether he'll take the work on or not depends on how busy he is at the time - there's usually more profit in working directly for a client, but there's a saving in not having to tender then wait to see if they take it up. So as I said, it doesn't have to be all one, or all the other - he does some of each.