I can imagine that you have a decent relationship with SS19, maryland...
How long have you had a relationship with him? What age was he when your R with you H began?
Forgive me if I sound out of place or rude here, but it would seem to me that SS19's knowledge of OW would be between your H and his X...
You have an R with SS19 no doubt... and it will be up to you to continue to nurture it...
He's 19 and can and will come to whatever conclusions he feels fits for him. And unfortunately, one of those conclusions might be that his dad is better with OW and that he (SS19) really likes OW...
Sorry if that's harsh... unless I'm somehow confused about you being M to your H for 3 years when the R was much, much longer??