Start with a few minutes at a time, and make it a silly game. And, as with so many things with toddlers, make a big deal of "oh, what a big boy" to help encourage. Pointing out all the other "big kids" he knows that wear a mask may help as well.
The first time we took our kids to see the mouse, my Mom explained to them that they would get tired and may want to whine or cry. However, she explained that everyone who works at Disney reports back to Mickey each day on any kids who misbehave. If you act up badly, Mickey may send a letter when you return home that says that you aren't welcome to come back.
My DD started to have a meltdown when she didn't get soaked on KRR. My DS reminded her that she didn't want to get a letter from Mickey and she immediately stopped crying.
Practice with the mask at home, but you may want to explain that Mickey requires all big boys/girls to wear masks in the parks, with a similar reporting back mechanism that will make him want to comply.
Practice wearing it at home-- wear yours too, so they see that its' normal. Have them wear it during times when they're otherwise occupied playing, drawing, etc. They'll be less likely to fuss with it if they are busy.
If you go for walks/play outside have them wear one there as well. Get used to having one on in the warm (assuming it's warm where you are!).