Was the previous felony related to drugs?
This same thing happened to me in NY, I got pulled over and the cop asked if anyone in the car had been arrested. We all said no. The cop comes back and asked me out of the car. He brings me back to his car and tells me my buddy had previous charges for possession of marijuana. So there you go, he has every right to be suspicious. A previous offense and a lie about it. I told the cop do whatever you want, search everything, I have nothing to hide, and I have no reason to protect my friend at all. The cop went back to my truck, talked to him and ended up letting us go in the end.
No offense to you, but you do sound like one of those fathers that believes his son can do no wrong. Not that your son did anything wrong (from what you told us) but I do believe that he knew something. If the cousin had that felony for weed or anything drug related, theres a good chance that there was something in that cigarette pack. And yes your son should be nevous because its his first pull over, but in the cops eyes your kids stoned and paranoid.
Its all speculation at this point, but kids with stuff on their record dont get it by accident and they dont necessarily learn from their first minor mistake either. And your kids at the perfect age to be experimenting with stuff like this.