Work on trigger control and follow-through. Shoot slowly. Snap caps and dry fire (look up the pencil drill) are great practice tools. A .22LR pistol will really help you practice for a fraction of the price. You'll bring those groups into the X in no time.
I disagree with Sigma40, I don't think stance should matter whatsoever in your ability to put accurate, rapid shots on target. Unless you're shooting Olympic-style target competition, the last thing you want is to be "locked." What's important is
repeatable trigger manipulation and front sight tracking, through the break and through the recoil, such that you visualize the front sight following a predictable pattern and it
returns to your index without the use of muscle tension.
Don't get hung up on technique beyond the basics. Shooting a pistol is a relatively simple task. Let your body perform by being focused in the moment without a cluttered mind. It's non-intuitive, but my bet is that
your son is doing this naturally, because he hasn't absorbed the wealth of knowledge you likely have becoming an accomplished rifle marksman.
Some suggested reading:
Practical Shooting: Beyond Fundamentals, Brian Enos
http://www.amazon.com/Practical-Shoo...9648679&sr=8-2
Or
http://www.brianenos.com/store/books.html, he has a number of excellent books available. I would consider him my go-to author for pistolcraft.
Surgical Speed Shooting: How To Achieve High-Speed Marksmanship In A Gunfight, Andy Stafford
http://www.amazon.com/Surgical-Speed...9648722&sr=1-1
T.A.P.S. Tactical Application of Practical Shooting: Recognize the void in your tactical training, Patrick McNamara
http://www.amazon.com/P-S-Tactical-A...=1CL07PD0KL43J
Tactical Pistol Shooting: Your Guide to Tactics & Techniques that Work, Eric Lawrence
http://www.amazon.com/Tactical-Pisto...9648953&sr=1-1
The Gun Digest Book of Combat Handgunnery, Massad Ayoob
http://www.amazon.com/Gun-Digest-Boo.../ref=pd_cp_b_1
The Gun Digest Book Of Concealed Carry, Massad Ayoob
http://www.amazon.com/Gun-Digest-Boo.../ref=pd_cp_b_2
Stressfire, Vol. 1 (Gunfighting for Police: Advanced Tactics and Techniques), Massad Ayoob
http://www.amazon.com/Stressfire-Vol...u-wl_list-recs