Anger is an extension of sadness. They say at the root of all anger is sadness.

Have his teachers said it's a concern?

Have you looked at the kids books about emotion? "When I feel..." I think they are called. It might be helpful for him to understand what sadness is and then he can let you know if he has felt it, even if he hasn't outwardly shown it or reacted to his own sadness iykwim?