Hi W.~
Most importantly, it seems that your son needs to learn that the behavior of running off is not acceptable and very dangerous. There must be a consequence for this. I think it's a good idea to take a stroller in the event that he does run off. Put him in the stroller for a period of time, no questions, no reservations, regardless of his protests. He needs to learn that that behavior is unacceptable. Until then, you will never have a peaceful outing with him.
In the meantine, though, I have three suggestions for keeping him safe while he is out of the stroller, enjoying the fair.
First, we have the Mommy I'm Here Child Locator. http://www.mommyimhere.com/
It's a little plastic teddy bear that the child wears, and you carry a key chain activator, and if the child wanders out of sight, you can press the activator, and the bear emits a loud, high pitched beeping sound, that helps you locate the child. Additionally, God forbid someone has grabbed the child, the beeping would call attention to them and hopefully stop them from proceeding. Obviously, this isn't to be used as a substitute for keeping a constant eye on the child, but it does give you a sense of peace in the terrifying event that he does get out of your sight in a crowded area. We have one and have never had to use it, but it's there for our peace of mind. I would just suggest familiarizing you son with the sound it makes, because it's loud, and tell him what it means and why he is wearing it. The website has a lot of information about it, and where you can buy it.
Secondly, we also have SafetyTats.
http://www.safetytat.com/
These are tattoos that you can buy that have "IF LOST, PLEASE CALL" and your phone number printed on them. You can buy them pre-printed, or ones that you can write on with a special pen. They also have tattoos for severe allergies, or medical conditions, so if your child is lost, it gives people information to help them. We also have these and love them. We have the "IF LOST" ones, and peanut allergy ones.
Lastly, I know many people frown on the use of child leashes, but that is an option. They do have the animal backpack-style ones that the kids tend to like, but that is a personal decision, whether you would use one or not. Personally, I wouldn't use one for an everyday trip to the park or mall, but if in a large, crowded area, and the child is prone to running off, it may be worth considering. I used one with my child a couple times when he was a toddler. You will get some looks, but it's up to you how you feel about it.
Best of luck to you. Hope some of this info helps!