Originally Posted by Crystal:
That being said....is it possible for you to spend the 15-20 minutes of nap just BEING with him? Lay down next to him, hold him, read him a book?
I think he just wants his Mommy, all to himself, and he doesn't know how to communicate that to you.
Find some time, when all the other kids are down, to give HIM 100% YOU. 
I don't have kids of my own but I recently started keeping my 4yr old up for an extra 20-30mins at naptime to work on some kindergarten skills. We do some work sheets or play a board game or some other simple activity that's hard for him to do when the younger ones are awake. Before he goes to bed we sit in my chair (kids aren't allowed in my chair so that's a treat) and read a chapter of a novel together. He wasn't having behaviour issues but he did need some work on some skills for school in the fall. He absolutely loves that time with me. He loves the activities is so proud of his own progress but I think the one on one time with me has been more valuable. I really enjoy that time with him, too.