Everyone has given you great advice. Whatever you plan to use, I suggest you test it out on the fabric you plan to use. I recently purchased a set of colored Bic and Sharpie permanent marker pens. Each set had about 6 or 8 colors. I was planning to use them on fabric for my quilt labels. As I wrote on the test fabric, I found they bled into the fabric. That is not bad if you like the affect, just make sure it is what you want.
I agree that the micron pens are hard to write with. You might want to have your son try writing his name on fabric once you decide what you want to use. Hope it all goes well.
It will mean so much to your son's teacher.