Something is off about my son's step Grandfather - HELP

I desperately need to vent about this and seek an outside opinion

I've always gotten a weird vibe off my partner's stepdad but eventually let my guard down a little because everyone else seems to trust him and think he's a good guy.

But my partner's mom said something that made my blood run cold when we were picking him up last Sunday after they kept him Saturday night. She said that the grandfather (step grandfather, not even blood) slept in the kids room with my son the night before.

Since then i've been trying to gently talk to my son and get him to open up and what i've gotten out of him so far is he tickles him all the time and my son wants him to stop, and that he hasnt touched his private parts but he has touched his butt over his pants for longer than a quick pat - "1 to 5 minutes" is how my son put it.

I know in my heart of hearts something is really wrong here and we have to cut contact with them. So i'm not really looking for advice, I guess I just want to hear other people validate what my gut tells me. It will give me courage because this is going to cause a serious fucking shit storm when we tell my partner's mom my son cant see them anymore.

This is serious, red flag behavior, right?