I found Play-Asia.com to be one of the cheapest at $32.61, https://www.play-asia.com/minecraft-java-bedrock-edition/13/70g2e5 (regular non-affiliate link).

It was the cheapest site that didn't seem dodgy / have comments online about it being dodgy.

Like anywhere you purchase it that isn't Microsoft's own store, you'll get a key that you need to redeem in your Microsoft account. You'll need a Microsoft account to attach the game to, a Microsoft account you're happy for your son to have access to. It looks like https://www.minecraft.net/en-us/redeem is where you'd activate the code.