Best gun for a child (13) to learn to shoot a pistol?

My son (13) wants to get into competitive shooting. He is 5 foot 3 inches tall and 100 pounds. After a range trip we have narrowed it down to pistols in: .22lr, 9mm and .40 S&W. We have also narrowed it down further to the Glock 19/17/34 and the S&W M&P in the large frame. All of the other guns we tried were either too big (1911's) or too small (smaller frame S&Ws). <br><br>We are ready to buy him a pistol and am looking for any input. How do you think a 13 year old will take to a glock and a S&W. I am leaning towards a Glock 34 (9mm) as his first pistol. <br><br>What do you think?<br><br>tl;dr: Best pistol for a 100lb 13 year old to learn to shoot competitively in .22lr, 9mm and .40 S&W.