I think in particular the comment about sucking cock was probably the thing that made flipping that much easier when his kid was facing mandatory fed time for dealing X.

There's another scene where they're in the safehouse where Carlo says they're going to meet Paulie at the hospital to visit Sil and asks Tony if he wants to come. He gets up from the table immediately and says he's got shit to do as he goes to lay down upstairs. Benny shakes his head and says "Yesterday it was his gout...." and Carlo is shaking his head disappointingly. I don't think towards the end Carlo got a sense that Tony had a lot of loyalty towards his guys. He obviously doesn't know Tony did in fact go visit Sil in the hospital.

A lot of things there probably made Carlo think "why not?" but ultimately it was his kid getting pinched that led him to flip. I'm sure the Feds leveraged the shit out of that situation in their favor. Ha.