I have one quick question for you. You wrote that you own a Glock 19 in .45? There is no such thing. The Glock 19 is a 9mm weapon.
Anyway, back to your question. If your son would prefer a semi-auto, I recommend the Glock 19. Not too small, not too big and it shoots cheap 9mm rounds. It is easy to take care of and fun to shoot.
If he is interested in a revolver, I would recommend a Smith & Wesson K or L frame .357 Magnum. This way he can shoot your .38 Special ammo, but also "move up" to full .357 loads int he future if he wants.
Last but not least, you are one heck of a dad. Wanting to buy your son his first handgun is pretty cool in my opinion. Maybe you can take him shopping to pick it out. That might be even more meaningful to him.