Sounds like the kid needed a friend and he thought your son might be someone he could hang with.
Instead of just answering his questions and allowing him to control the conversation and obviously elicit an emotional reaction from you in the form of apparent frustration, you should have been the adult, assumed control of the situation, and began asking him questions.
Oh, hi, I'm Mr. Johnson, what is your name?
Where is your campsite?
How long have you been here?
Where are you from?
Do you have any brothers and sisters?
Are you here with your mom and dad?
How old are you?
Do you have a dog?
Do you help your dad set up your campsite?
This is my son Cletus, he has a bike too. Why don't you stop back later and maybe you could show him around after we get set up.
Things like where he is camping, where his parents are, any siblings, where he is from, how long has he been there, etc., and you would have figured out right away where things stood with him. Instead, you immediately concluded he was a pest and treated him as such.
There are so many ways you could have taken control of this situation and turned it into a positive, but instead, you wrote the kid off as a pest right off the bat and then treated him poorly.
I say "shame on the OP" for handling the situation like a child. He was just a kid on a bike...and we've all been there.