Easy! Tell him to go to Home Depot or Lowes and ask for "onion starts" in the garden center. Then, either plant them in the ground, or in a pot, so the top (pointy end) of the onion is just covered. Root end goes down (G).
Water when the soil looks dry, and he will have fresh green onions to harvest in a very short time. If he wants large onions, tell him to leave them until fall, then pull them up. I much prefer using the green onion tops, myself..

I grow them all year-round in my garden beds or in a container.

Lisa