I struggled with this issue last season (2010) when my son was 8. He was throwing with 3 fingers but, for some reason, started lifting his index finger when he threw - thus basically throwing with his middle and ring finger. I couldn’t get him to stop lifting the index finger, and so I switched him to the correct 2 fingers: middle and index :lol:. His control was not affected much, but his velocity noticeably increased.
Your son will see an increase in velocity when he goes from 3 to 2 fingers.
The one thing I did do when I made the change from 3 to 2 fingers was have my son long toss with his new 2 finger grip. That got him comfortable with the new grip quickly. You may wish to do this when your son goes to 2 fingers.