OK ,9yrs old is a little young,but thats ok.Trap is a 12ga game,and anything else puts you at a very real disadvantage.Skeet can be shot with any ga. and shot well. So ,I would recomend ,a gas operated 12 ga,with good 1oz loads,and interchangeable chokes.A 28 inch barrel would be my choice with a vent rib,single bead.The most important thing at this age is that the THE GUN FITS.Take the time to take him to a good gun shop and have the stock measured and fit to your son.Later you can add a Morgan adjustable recoil pad. If the gun doesn't fit he realy doesn't have a chance, and he will learn a lot of bad habits, that will be hard to correct later. He will get frustrated, and convince himself that he can't do it. Agood fitting gun is a joy to shoot,tames recoil, and gets you on target faster and consistantly,which is the most important part of the whole game.