Wow. I am sorry. But that is borderline out of line. Every relationship is different. Just because she is not sexually active on a regular basis does not automatically mean she is cheating. And to suggest he needs a DNA test on his son. Harsh.
To the OP. My husband and I have been married for 11 years and we are working on rekindling our romance. It has been up and down. Sometimes we have long lulls. We both have confidence issues, I won't go into it here, but I do want you to know that you are not alone.
Maybe counseling will help? It helped us. Also, it is hard to "start over". Once you get in a rut, it is hard to get out. I saw a program on TV where couples committed to having sex everyday for a week to jump start their sex life. Maybe you guys could try something like that?
Good luck. Communication is key. You guys need to have an open dialogue.