How do I bring this up to husband?

So I made a post a few weeks back about how my husband and mother in law forced our 3yo to go to her house for 2 nights. He was crying yelling how he didn't want to go. And then when he came back, she "decided" how my in laws will take him every month for 2 nights at a time to get him "over his clinging to me bevause its unhealthy". When he came back, and I was cuddled up with him, he took my hand and would open up his pants amd shove my hand down his pants. Hes never done that before. I mentioned it to my therapist and she said I need to keep him away from the in laws, and if anything they should only have supervised visits with him. She believes either they, or someone they had around did something to my son. I told my husband today there will be no sleepovers anymore and he said "ok". I explained how our son is starting to build a bond with his sibling and he will view that as us abandoning him ans will cause issues. (Therapist stated that too). But how do I bring up about him possibly being molested by his parents or if they had him around anyone who did? My therapist is reaching out to others to get their opinions and once she gets back to me with their opinions I plan on bringing it up with my husband. How do I go about bringing this conversation up?