I know you are not supposed to promote your own business on this site, but I just came up with a foolproof idea. For the low, low price if $199 plus S&H, you can purchase my old bucket of baseballs (approx 40) that come in various weights ranging from around 6oz (probably not regulation size) up to maybe twice that (depending on when it last got rained on). This offer is only good to the first caller at which point I will begin putting together another bucket and make it available next year. Have your young pitcher grab a ball and fling it - I'll think maybe I'll trademark it as the "Vari-Weight-Ball" program and make it available only to prepubescent kids and then report on velocity increase once they start shaving.
On a serious note, what it the allowed variance in official baseballs - high school, college or MLB? A think I once read a variance of only 0.2 oz was allowed but that got me thinking about how much difference the plus/minus 1 oz makes. When do kids take special care not to start throwing waterlogged ball that might already have that extra 1 oz. I know my 2017 has a few baseballs that he keeps hidden from his younger brother.