keyboard not using keypad

My son's account on our iMac at home has this persistent problem with the keyboard.

On my wife's and my account the keyboard works fine. On his, the key pad keys do not function.

This makes it very irritating. But I don't have any clue what is making it do this.

There is no setting in the keyboard/mouse prefs that change anything.

We are currently on Yosemite, 10.10.3, but this has been going on for quite a long time, probably before Mountain Lion.

iMac (21.5-inch, Mid 2011), 24 GB RAM, 2.7 GHz Intel Core i5,

I have already trashed his Finder prefs, keyboard layout prefs. No effect.