Paul is both a veteran as well as a felon. He started using and abusing alcohol and drugs at an early age. It went from pot to pills to heroin.  His disease progressed, as it does, and after receiving a DWAI he had a wake up call and decided to go into the Air Force to try to get back on track.
Things got better but he quickly fell back into his old crowd and old behaviors. Heroin got a hold on him again. 
That's where the real trouble started. Paul had multiple serious bouts with the law that resulted in serving time in prison. 
Once he started losing his friends to addiction, he had another wake up call and this time it stuck and he chose a life of sobriety.