Adding extra weight to any normal motion in sports should be done slowly. I would not advise weighted balls solely for the purpose of strengthening your sons arm. The only way to strengthen your arm for throwing is simply...by throwing. Long toss is great. I learned this the hard way last year. After using weighted balls to play catch with my older brother, My arm motion changed when I threw the lighter baseball, and I began to let my hand get under the ball, making my throws wild even from second base! Took me a while to get my throwing motion back.