I was also a child who began wearing glasses at a young age due to astigmatism. I realize the common realization is that children will wear their glasses once they get used to them, but I believe you wanted to know how to get to that point. First, help your son pick out a pair that he likes, as was suggested by one of the other respondents. If your son becomes involved in sports leagues, you may need to check into getting special sports glasses that are strapped to stay on his head while playing sports.
Your insistence and persistence in having your son wear his glasses is the most effective way to get him to keep them on his face until he gets comfortable, but perhaps you can give him incentives for going the whole day (and later the week) with wearing his glasses without your insistence. I recommend books (reading, coloring, stickers, etc.) since those items require good eyesight (thus reinforcing using the glasses) and I have yet to meet a kid who doesn't like books, especially when they focus on a favorite topic or character.
I hope your son does outgrow his condition, but don't be discouraged if he does not (I was one who didn't). Lasik procedures exist to correct astigmatism which is what I had done a few years ago, and I am glad to be glasses-free.