C.J.
Hi L J
The first thing I would do is sit him down and talk to him. Ask him if anything has changed at home that would cause him to miss his mom so much. It could be that his mom has a guy friend that your step son has not become use to. And he may feel as though when he is at your house that his mom is spending time with her friend and not thinking of him. I know this sounds crazy but when children are at this age and their parents start taking an interest in someone, the children some times act out in strange ways.
Try to get him to talk without making him feel as though you are prying. He may not want to come right out and share information with you. Try asking him if there was something that he wanted to do at his mom's house but couldn't do it because he had to come visit his dad.
See if there is anything going on at school that he will discuss with you. Behavioral changes can be very stressful, especially when you don't know the cause of them. If you have to, go to your step -son's school and speak with his teachers. Ask them if something may have happened that you don't know about. If possible don't let him know that you will be going to the school. Kids don't always like for parents to know what is going on at school even if they don't know how to handle things themselves.
I hope this helps. Be blessed!!!