So sorry you are going through all of this...:)
At least all the Mama's here will offer you their best comfort...
I just wanted to add... than in addition to the caring and love you are providing... it may be best, to get your your son some child counseling. The ripple effects of divorce can go on and on...even years later, and affect the child.
My friend, went through a divorce when her children were 2 yrs. old and 3 yrs. old. She sought counseling for herself... (so she could Parent them in the best way possible through this trauma), and child counseling for her children. Yes, divorce is a "trauma" event for a child.
Her children benefited in so many ways, from the counseling. They went for 2 years... and through this, it reduced the emotional and behavioral problems that the divorce created. It solidified them...and provided stability and coping skills for her children. THIS was the smartest thing she said she did for her children. As a single Parent, post-divorce, she simply could not address ALL her children's needs and help through the divorce. She too was a working Mom... so it increased the hardship for her. But with the regular counseling for herself and her children... it really brought closure and "peace" to her children, and addressed issues of abandonment, self confidence, etc.
I would really recommend counseling... it will navigate the whole "problem" for you and your dear son. Remember, it is a process... making it "all better" will ebb and flow.. as your son is adjusting to all of this... and it can take time, or years. At least your Ex seems to try too, for your son.
Especially for "boys" as well...the need to learn how to feel "safe" for expressing themselves and confident about it... and secure.
*Just something interesting: I have a 5 almost 6 yr. old girl, and when she was in Kindergarten, she would come home and tell me about her friends. In particular... she would say things like "Sally's Daddy doesn't live with them...her and her sister and brother live only with the Mommy.... the Mommy has a hard time with all the kids....The Daddy lives someplace else in another house....and with somebody else..." I would explain simply that not all Parents live together, but they are still a Mommy & Daddy etc. Gee, but it really shows that kids, even when very young... are "talking" about this already to their friends and such. They are not clueless and really are affected by things... if anything.. even the more reason to help them and/or via counseling. Kids this age just need professional guidance to cope many times. It seems, my daughter's friend in school, would vent her feelings this way, telling her friends about it.... what an eye opener!
All the best...
Suan