It takes a while. Given that it appears you are already into a mite program, it's a little more complicated, because you don't want to quit in the middle. But learning how to skate first is the best suggestion... in a class and/or public skate... and a no pressure hockey class/clinic before getting on a team with peers and team expectations. Skating and skating with a stick and puck handling are a lot at one time... learning to skate is enough of it's own at first. Really.
My younger one knew how to skate and wanted to play as a mite (anything his older brother does he wants) but then hated it so much his first mite season (just couldn't stay focused out on ice... ) it became a battle to get him to practice and dressed for the ice that I let him quit half way through that first mite season after a few lectures about not quitting in the middle (quitting at the end of the season is OK if it's not your thing). He started again as a second year squirt, and is happy to play now, and is progressing well.
At this point, he's frustrated and discouraged, and it's as more about maintaining a positive attitude by letting him know there is no pressure and you just want to help him learn and enjoy the game, but get him to pubic skate as much as you can in addition to his hockey practices... and if you or one of his friends or someone you know can get on the ice with him, it will keep him more interested... or sign him up for some skating sessions... while it may sound expensive, it may only take a few at this critical point of his attitude formation to build his confidence. You have a great foundation for good attitude in that he was driving the decision to play in the first place, it wasn't something you dragged him into.