Bite inhibition's not hard, its just repetitive and can seem to take forever.
I'd start by hand feeding him a large portion of his meals. Just the adults first, till you're sure he's figuring out how to do it without snagging fingers (palm and fingers flat, like you're giving a treat to a horse, and put the kibble in your palm, if he's rough grabbing the bite close your hand over it, he WILL chew on your fingers at first, till he licks your hand then he can have the food), and then have your son put his hand inside yours and help feed the pup. Impress on your son that he's only to do it when you can help him or one day you'll find he's pulled the bag of kibble out of the cupboard to feed the dog.....I remember being 5 lol.
When he nips or mouths in play you need to do two things 1: yip a highpitched ow! and then 2: walk away. He bites you go away, this may mean just moving seats, or even leaving the room depending on the situation and whats going on. Just a note on the yip, some pups get more excited when yipped at, in those cases a loud OW! in your normal tone of voice often does the trick instead. He's also more likely to get mouthy in play when he's overly excited, so keep an eye on him and when you see him headed that way stop the play and do calming things, or put him into a short timeout in a puppy proofed bathroom or the like. And remember it does take time to get it through his head. They have NO attention span at this age and so much just goes in one ear and out the other. But he WILL catch on, I promise!