Ear muffs would be your best bet for him. Take him with you to buy them and try them on him, be sure the band adjusts to the correct size for his head and check the seal all around the ear. His ear should just fit inside the cup.
You should be able to find some youth sizes that will be fine.
Like Ian said, a sporting goods/firearms shop would be the best place to look.
And goodonya for watching out for his hearing
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