Absolutely tell him. I struggled when my 18 year old kept asking me if "Dad had sex with the other woman." I KNOW he knew this answer, but he really wanted to know. I told him to talk to his dad, but now unfortunately he and my 21 year old daughter know WAY more than they should
I wish you luck. Sounds like your little guy really wants to know, and information/truth is freeing sometimes. Good luck!