As a coach in a Youth league, I would definitely put him into a youth league. Most centers will group the kids by age. Talk to your local center, the amount and type of youth leagues varies center to center. Some cater to the older kids, some have a wide range of options.
One thing I would ask is if they have certified coaches. My reason for this recommendation is I've seen youth leagues where parents do the coaching and, while they may have the right intentions, they do more harm than good to the kids.
I would also advise the Junior Director that your son already bowls without bumpers. Most 4 year olds are started in the Bumper Leagues so they can learn the basics without the penalty of gutter balls. As the child relies on the bumpers less and less, he/she is moved up to the next level of league. In my area most of the leagues for the little ones are non competitive. They keep score and bowl 2 - 3 games, but in the end all the kids get trophies of participation with no league champions.
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USBC Bronze Coach "I cannot change the direction of the wind but I can adjust my sails to reach my destination." Jimmy Dean
Quaker 10/93 - 4/07
Quaker