Hemmi it might be fine now but it is simply bad mechanics. Like having a wheel out of line. It may not be too much problem when going slow but on the highway it is really bad.
I will share a little story.
About 5 years ago a NCAA asst. coach who was in charge of recruiting and used to stay at my house when he came to this part of the world, was here to look at a Jr. A defenseman. We went to the game about 2 hours from here and he had the same grip your son has. He was a great skater and really smart player and you could see that he was trying to hold his stick in a mechanically efficient way but it kept slipping back to poor mechanics. His passes and shot didn't have the zip that an 18 year olds should because he couldn't snap his wrist in the direction of the projection. He wasn't offered a scholarship because of this.
If your son has any aspirations to play at a higher level he should tape his top hand to the stick until he gets rid of this ineffective grip. If he just wants to go out and play he will never be as good as he could be.
Thanks very much for that story. Just want to think that I was being harsh by taping his hand but in order to get it right that is the only to force the memory. Thanks again!