It is sad that candy is everywhere, and I agree with HealthyMe2010 about rewards giving kids the wrong incentive to do something.
Pacificfords, I love your plan for having your son trade in his candy rewards for something vegan and gluten-free at home. You can make healthy treats to let him trade for too - it doesn't have to be junk food!
Healthy eating is one of our "family values" if you will. My 12 year old is on board with the McDougall way of eating (in our case, we have the added restrictions of no gluten, no corn, no soy, plus mostly organic) because (1) its all he's ever known and because (2) we've talked about healthy foods and healthy bodies a lot. We make healthy snacks and treats, so he's not deprived! We homeschool so that has helped a lot, especially when he was younger! We don't run into the food as reward thing so much as every darn gathering is a potluck or every soccer game requires a snack, etc. We just always make sure to bring (or send) something he can have.
ETA: If my son wants to make banana ice cream for breakfast, he's more than welcome to do so. He usually makes himself gf oatmeal though or his version of Mary's oat waffles.