Why was the sheriff stopping him in the first place?
My best advice is to not take legal advice from people on a forum!
It appears to me that there is a lot of information missing here. I just have a hard time visualizing the sequence of events that has a kid rounding a corner, a sheriff having the time to recognize a violation and then get off his quad and tackle a rider. Something just doesn't add up here.
If there is validity to your statement that your son didn't realize it was a sheriff then I would just go to court and explain the situation to the judge. If this is just a citation then I would expect the judge to throw it out or at least reduce the fine. Was there any other citation other than eluding?
How old is your son?
If the fine is hefty or it is more than a citation then get yourself a real lawyer.
Rod