He has to trade with someone who owns let's go. The game is intended for you to trade with friends and only friends doing a code system.
But you can still transfer pokemons from Pokemon Go to Pokemon Let's Go. I got all of my exclusives that wasn't on Let's Go Pikachu from Pokemon Go app.
OP: Can't you get your son to get you a Mankey and get him to transfer it to you like he did with the others?