D.P.
Awwwwww...what a sweetie pie.
I've always thought they needed to have therapists available for the moms of Kindergarteners!
All I can tell you is that is does get better. (Or at least it did in my case!)
Make your together time as special as you can and just let him do the things he is missing.
Sometimes, what I do (mine is now in 2nd) is get him something special (just little things) and leave them on his bed to find when he gets home.
You give your kids roots and wings and this is the beginning of the wings part! (The hard part!)