I've been on the "least favored" side both as a child and as an adult. It is tough. The one thing that I remind myself of is that I can only control myself, and the person that I strive to be is one who is bigger than all of the petty stuff. It still does get to me sometimes - sometimes I feel a bit like a doormat. But I try to remember that I am happy with the choices that I've made, and I don't think I'll ever regret rising above it all.
Your son will likely eventually realize that he's less favored, but it could take a long time. Honestly I didn't fully realize that was the case with my grandparents until I was an adult - there were many clues yet they were good about hiding it. For your son it may sting in the short term, but in the long run it really is their loss.