Just something to add
I am new here, and I guess this topic has been pretty well covered, but thought I would add my two cents.
As a poly woman and mom, I understand your worries about this, it can be nerve racking. I feel, just my opinion, that hiding such a huge part of your life from you kids can be very hard on parents as well as kind of impossible. It's likely kids will find out eventually. I'm not saying you have to spill all the beans at once, but it may be time to start sharing with him how you view sex as an adult so that when he does find out it is not a devastating shock to him,