Jack is not really sure at all where he intended this to go next.

"So... so if you're gonna try and get revenge for him--"

Revenge. He might have missed out on a lot of things that would win you a Father of the Year award, but Fontaine taught him all about revenge.

"--I can stop you."

He doesn't sound like he's about to start blazing his guns at Sinclair, but it's definitely a threat. A sickly-sounding threat, admittedly, but not an empty one.