The stepmom

My son is at his father's for the week... He didn't want to go. He doesn't like going over there too much...  He loves his dad and his family over there... but he feels that his step-mom is mean.  
I am not really sure how exactly to handle this situation... 
Lil Man has told me about it, my bf, my parents... but not his dad.  I've talked to his dad a little bit about it when his dad has came out and asked me why Lil Man doesn't want to go over there... but that conversation didn't end well...  He was very defensive. 


I keep telling Lil Man that he needs to talk to his father about what bothers him. He said that his dad already knows... I've tried to explain that just because I have told him.. It doesn't mean much. That his dad may think that I am just saying it so Lil Man doesn't have to go over there.. How do you explain that to a stubborn, hard headed 8yr old? 


His father has pretty much told me that if Lil Man doesn't want to come over... don't force him. I told him that Lil Man is not old enough to make that choice. He's 8! And that maybe he should sit down and talk to Lil Man about it. 


My heart goes out to Lil Man and his dad.. The situation is just not a good one.. but I really don't know what else to do...  I want to make it better for everyone.. but I really don't know how.  I don't want it to get to the point where my son starts acting out or something because of this. 


His father loves his very much and wants to see him as much as possible, which is fine with me... but I hate forcing him to go over there when he feels like he does. 


Any suggestions? 


** Lil Man's dad has a step daughter who's 6 and a son's who's 4
*** Lil Man's dad and wife have been married for 3 years
*** Lil Man's dad and I have been divorced for 6years
**** Lil Man's dad and I have a decent relationship