Cheating bf caught how to approach

So I recently did some digging and found out my bf who has a child with someone else is actually living with the mother of his child. I asked him previously before do you two stay together and he has told me no. I have been to his place that he shares with his brother so I was never concerned. I did some google digging and found out on true finder that they both live there.

I want to bring this up I 100% plan on leaving him. But how do I bring this up without seeming like I was stalking or obsessing I certainly was not