Hello C.,
I'm sorry for your loss and your son's loss. My exhusband/son's father passed away last winter. It's very hard.
There are some groups around columbus that help kids deal with the loss, so I bet there are some in your area too. You might want to call local hospitals and hospices to see if they have any suggestions.
The hardest part that I found was to deal with the transition of how I had to view the ex. Even if you and the ex didn't get along, there still was something that made you get with him in the first place. After my ex died, it was hard to put away all of the negativity and start saying good things about him, but for your son make sure he knows the little things that made his dad special. After almost a year, I'm now saying, "Oh, your dad used to do that" and telling stories about our misadventures so that my son will always know his dad in some sense.
Of course, there's the fact that some kids deal with it in different ways. My son didn't express much emotion about it, but that doesn't mean that he kept things bottled up-- it's just how he dealt with the shock. For about 6 months afterward, he was very touchy when playing with his friends and wanted to be alone sometimes. I gave him the space to do so, and gave him hugs and listened, without being pushy about him sharing his feelings.
Be sure to get a hold of social security. Your son is entitled to your ex husband's social security money, and it's a real help.
Good luck and be sure to take care of your emotions as well as those of your son.
J.