A.V.
You can also ask the teacher (or pediatrician) about how he grips things. Does he have problems with coloring and scissors, too? My DD was grabbing crayons with a fist grip last year and they suggested I work with her to form a pencil grip. It's made a difference. And, frankly, I don't make her write a particular way. If the lines get on the page and it looks like a letter, it doesn't need to be "standard".
A fun way to practice writing is to use bath crayons or bath markers.