Susan Fridie's List: Keyboards & Mice

The setting you want to change is "Ignore quick keystrokes or slow down the repeat rate." You don't need to "ignore," you need the "slow down" part. After you click the box, be sure to hit the "Settings" button right next to it.

Inside the Settings box, suggest you start with
1) Check "Slow down"
2) Set "Repeat delay" for 1.0 secs
3) Set "Repeat rage" to 0.5 secs
the close the dialog boxes.
When he tries it out, if he's hitting a key and getting no letter at all, then make the Repeat Delay less than 1 second. If he's still having trouble with getting strings of letters, change the "Repeat delay" to be longer than 1 second.
Once he chooses a setting, have your son try typing with these setting for at least 1/2 hour (maybe 15 min each for 2 days), before adjusting them again. It takes a little while to get used to each change.