Projects that don't work as expected make the most interesting science fair projects. This is a mystery that your son can investigate using a scientific approach to find out why the maple sugar did not crystallize. A careful analysis of the problem will make an outstanding project.
Since your son has been able to make crystals using a simple water and sugar, his technique in boiling the syrup has been eliminated as the source of the problem, and the pan used for the experiment is obviously not coated with oil. This means that he has isolated the problem to the maple syrup; there is something in the sample itself that is interfering with crystallization. The two primary possibilities are that the syrup could have too much acid in it (low pH) or it may not contain enough pure sucrose.
First check the label of the maple syrups you purchased, and check to make sure it is pure maple syrup with no additives. If anything (acid, enzymes, or corn syrup) has been added, then the syrup will not be suitable for crystallization. If the product has a lot number on it, you could try checking with the manufacturer to see if they have a record of the analysis of the lot, including the pH and the Brix (sugar content). This information would be helpful in solving the problem.
To verify a pH problem, you can test the pH with a pH meter or pH paper, if you have these available. Or try adding a small amount of baking soda to neutralize the sample. See if this will solve the problem. Pure maple syrup will have a pH of 6.8.
Sucrose, the main sugar in maple syrup, can be converted to its individual sugars, glucose and fructose. If too much sucrose is converted, then the syrup will not crystallize. Maple sugar that has a Brix of 66 degrees or greater is best for crystallization. A lower Brix product would not crystallize well. The Brix of the syrup can be measured using a refractometer:http://www.eckraus.com/RF110.html
At $79, I would not recommend getting one of these for this project, but you may know someone who has one you could borrow for an experiment.
Please let me know what your son discovers. And let me know if you need more explanation on the science or the analysis of the syrup. I'm very curious to know what you find out.