Did the kid get on the bus? Probably a high school student unfortunately. Definitely worth calling the police but you may also want to contact the school and bus department and give a description of the kid as obviously the buses frequent the bus stops and the school may have an idea if the kid goes to school there.
Unfortunately I would say this has less to do with the area and more to do with kids just getting into too wrong of stuff nowadays. Even when I was in high school and college, lived in a good area, good part of town, went to a prestiges high school where most of the kids there were Honors and IB class kids and yet even that great of a school had a drug problem as did all of the schools and it wasn't uncommon to see younger "kids" dealing at the bus stops before and after school.
Very glad your child knew the right thing to do/say and hopefully this will help other parents to teach their kids the same thing.
(Also did you contact our Block Watch? As well as the school district? I would imagine both would send out e-mail and/or phone warnings to parents regarding that person especially since you have a physical description). I'd definitely contact the district and school on Monday and block watch and PCSO over the weekend.
And tell your son awesome job!