He'll only trust you with his feelings if he believes you can handle them. Try not to dismiss his feelings or tell him to change them. Instead, talk about why he might feel that way. Encourage him to recall specific incidents, then talk about those feelings. Offer your reassurance that he was not the cause of your marital problems.
Some children find it easier to talk with peers than adults.
If your son continues to keep his feelings to himself, check to see if his school, your church or family counselor offers group discussions for children in transition. What's important is that your child identify his feelings, express his emotions in acceptable ways, then learn to cope with those feelings. Even if your child isn't quite ready to talk with you, remind him that God will listen to whatever is on his heart. You might suggest he start a prayer journal where he can write down things he's praying about, thoughts he wants to share with God and anything else he likes. Kids at this age often want to take a physical action to solve a problem and a journal can be extremely helpful in the process of identifying and working through emotions.