My Girlfriends son may ruin it for us....or did we ruin it?

I'm recently divorced. I met an incredible woman. We are in love. The problem is that her son, who is 11, caught us French kissing. We were kissing hard, laying together, in the dark =, on the living room couch. Her son, who we thought was sleeping, was on the other couch - opposite from us. This was two months ago. He still brings it up - to his mom and his dad. And when I'm around and his mom and I hold hands or snuggle on the couch, he gets quiet and sometimes gets mad. If I'm not being affectionate with his mom, he and I get along wonderfully. He's a great kid. And as early as last week, his mom and I sat him down and explained things and told him that we were willing to have a "no-touch" policy when he's with us. Just to make him more comfortable. Well, he still says he has nightmares about what he caught us doing and has visions of me hurning his mom - which I would NEVER do. What else can I do about this? I want this relationship to work, with me,him and his mom