What's in it for me? Simple... Retribution. But he doesn't need to know that.
"Since I left the force, I've been bored out of my damn mind. I'm tired of seeing punks roaming the streets doing as they please while cops like me and you get fired or killed for doing their job. Hell, I might even be able to get back on the force if I play my card right. In any case, these kids are a bunch of fools. There's no way the Masen Twins are acting alone, and taking them off the streets won't put even the slightest dent in whoever is running the show. Let me get to the bottom of this and all of us will come out on top with our heads held high. I am and always will be an officer, with or without a badge or payroll. Though its more difficult to get supplies, being a civ makes it easier to blend in with the right crowd. If I came up to these punks in uniform I'd have gotten shot or simply scattered them to the seven hells. As a civ, I can pull strings, make connections, and put the pieces together like I never could as a cop. If you can help me, I can be your undercover informant and it'll be just like when we were both on the force, except I'm not held back by the all-seeing corporate eye. Understood? We can live in the trenches getting shot at, treated like garbage by the system, or we can climb our way to the top. What would you rather be, a cop on the beat your whole life, or someone with influence and a reputation? You're a smart guy, you know this is an opportunity you can't miss."
Also, get his contact info so we can chat when needed. We might be able to scavenge some supplies and intel from him in exchange for info later on. Doesn't even need to be info on these guys. As a shady sociopath we can easily get in with drug runners and every other criminal, cut throats on the way to the top, and topple all of our aggressors and rivals with our police connections.
LETS FUCKING DO IT!