I think you can mirror him and empathize with him regarding this relationship ( I know you find this boy to be an exciting, fun guy to be around and I can understand why he seems so appealing). You can then ask him to listen to your concerns (notice I didn't say condemnations) regarding his hanging out with this kid. Tell him you hope your concerns are unwarranted and tell him you think he's smart enough to know if he's being controlled by this or any other person. Watch for any behavioral changes in your son that worry you and keep connected to him.
Avoiding Peer Pressure
Toddler and Teenager Expert Advice from Carleton Kendrick, Ed.M., LCSW