Cleaning Question

Yes, water is fine for getting actual dirt off the coin. But be careful not to rub the surface and scratch it with the dirt.

Also, do a search here on acetone, so he can learn to use it properly.

Basically as noted. However, I suggest a small investment in a gallon bottle of distilled water. Usually only about a dollar. Place that coin in a glass dish, bowl, etc that is already clean and allow it to soak in distilled water for a while. Even a day will not hurt the coin. I mention distilled water due to some areas have numerous additives in our tap water that is actually harmfull to many coins. In large city areas Fluoring is used as a germ killing agent in our drinking water. If you have a water softener, the water from there contains NaCl otherwise known as Salt. As a matter of fact if you have house plants you should always allow tap water to stand for 24 hours prior to putting on a plant. The gasses of Fluorine or Chlorine kills plants. And never use tap water from a water softener on plants. As most people know Salt kills plant life.
Back to your coin. IF that coin after the water still shows signs of corrosion or dirt, you may want to try the Acetone method. However, there to this substance is highly flamable and if you don't need it around the house, I suggest forgetting the purchase of that. Although Acetone is available in any store that sells paints, it is not a really safe substance to leave laying around.
Ome more possible, safer method is to make a real watery paste of baking soda and that distilled water. Rub that on the coin very, very gently then rinse with more distilled water.
HOWEVER, the main thing to remember about coins is NOT TO CLEAN THEM.