Need help with bridging thoughts about step son having an affair

I have a 24 year old step son who I have only known for 2 years. I have only been married for 2 years, but he wasn’t talking to my husband when we dated. For the most part, we have had a good relationship. I enjoy his company, but don’t feel remotely parental towards him. He is sweet to my 2 daughters.

It recently came out that he had a 5 year affair with his ex girlfriend’s mother while he was dating the daughter and he was super close to her father. Their family has taken him on vacations all over the world. Her father got him an amazing job he wasn’t remotely qualified for selling medical equipment to him. I am so deeply disturbed by the fact that he essentially had a father-son relationship with his girlfriend’s dad while he was sleeping with his wife who was his girlfriend’s mother. The betrayal of both the daughter and father makes me question his judgment in every aspect of his life. I now don’t believe anything he says, and constantly look for holes in his stories. This is causing a problem with my relationship with his father, and I need to get to a place where I can be loving and supportive to his father, my husband.

Where I am:
C-step son betrayed girlfriend and her father by having 5 year affair with her mom-his wife
T-he is a lying, deceiving, terrible person who can’t be trusted with anything
A-judge him, withdraw me and my daughters to protect us from being betrayed by him too
R-hurt relationship with husband

Where I need to be:
T-He was a hormonal 19 year old
A-have compassion
R-we come together as a family over this

I need to get over my belief that he is a threat to me and my daughters. I know he didn’t betray us, but if he could carry out a betrayal of that magnitude for that long, I don’t know what else he is capable of. I don’t want my children exposed to him. What are some bridging thoughts to help me get to a better place? How do you have a relationship with someone who lies? I’m trying he was a hormonal 19 year old. He tells lies and that is ok. He is just someone we see on holidays and who goes on our vacations with us.