Wtf do I do?!

So my SO has a 9 year old son with another woman. He found out she was pregnant when they weren't together. There has been a lot of drama and we barely get to see his boy. I've developed a pretty good relationship with my SIL and she told me today that my SOs baby mama told her that she slept with his best friend when they were together. Now Noone in the family thinks his son looks like him and there has been doubts due to her promiscuity. The question is.. do I tell him and risk making him hurt/mad and ruining the trust I have with his sister or let him know. I don't know what to do 😣 if you think I should tell him is here any advice on how to word it? I'm not very good at this sort of thing!