I agree, gun fit on a shotgun is more important than action, make, model, etc for new shotgun shooters. There's nothing that can help or hurt a new shotgunner more than a gun that doesn't fit. I've seen people who show up on a trap range with a $5K over/under shotgun that doesn't fit, and they're very disappointed with their gun, which they were assured by the salesman was "a great gun for trap/clays." I've seen guys with a Remington 870 from the early 1960's that the shooter has modified the stock to really fit him and watched him bust hundreds of clays in a row, not a single miss. 

What most people don't understand about shotgun shooting is this: most shotguns¹ have no rear sight. Your eye is the rear sight. As such, your ability to aim the shotgun at what you're shooting, especially as you're trying to swing the shotgun along the target's anticipated path, depends on your eye being in the same position relative to the rib and front bead of the shotgun every time you mount the gun. A gun that is too long in the pull, too short in the pull, too much/little drop, too much/little cast-off, etc will result in not achieving the same cheek weld every time you mount the gun, and trying to hit targets that appear rapidly, requiring you to mount the gun after the target appears, will cause frustration, because you're trying to manipulate the gun, contort your face/head/neck at the same time you're swinging the gun, all at the same time. 

For most beginners, this is too much to ask for them to remember and get right while they're learning to swing. 

Most guns today are still made with stock dimensions for the "average" man of about 100 years ago. For many youngsters today who are over 6' tall, this average results in guns that might be too small. For most female shooters, this results in guns that are, at the very least, too long in the pull for the average sized woman, and much too long in the pull for a gal who might be between 4'10" and about 5'3".  If your son is taller/shorter than about 5'9 to 5'10", then he might benefit quite a bit from fitting. 

When mounting a gun, I like to see the shooter mount the gun to their "pocket" (of their shoulder) first, and get the heel of the comb line to the top of their shoulder and the butt in the pocket of their shoulder, then bring their face down on the comb. This gives me a better idea of how the person is fitting the gun and what adjustments need to be made. Most people who get the comb line fitted to their cheek first will compensate by pulling the butt into their shoulder "where ever it lands" - which might be too high, too low, too close to the center line or too far out on their shoulder. Then after about 50+ rounds of trap loads, they'll start to notice that their shoulder starts to become uncomfortable, because they're mounting the shotgun to their cheek first, and not mounting it for handling the recoil first. 

All of this is independent of the action/make/model of the gun. I've seen people need a $5K Beretta O/U fitted, and I have an old Fox Sterlingworth side-by-side that I found well used, all I did was adjust the stock a very little bit, it now fits me like a glove and I can smoke clays without seeming to try. The only downside of that old Sterlingworth is that the barrels are under-bored, and they become very hot after 25 rounds, never mind 50.

The second thing I have people do who are having "trouble hitting anything" is to pattern the gun. You would not believe how dramatically patterns vary from gun to gun, choke to choke... some cheaper/shorter screw-in chokes make a mess out of the shot string and result in lots of fliers in the pattern. Some older guns with fixed chokes can have very nice, consistently tight patterns for upland hunting that need to be opened up a tad. 

There used to be a gentleman and his wife who made a wonderful adjustable (for LOP and comb height) stock for the Rem 870, 1100, 1187, who was named Bill Davis. Sadly, he and his wife have both passed on. I don't know if anyone has picked up the business, but you might look around and see if you can find an old stock set from Bill Davis for a 870 or 1100 gun. They worked pretty well for the money they cost. Boyd's Gunstocks have an adjustable stock product, but it appears currently that it is addressing only bolt-action rifles. You might give Boyd's a call and ask if they're going to adapt their adjustable stock for shotguns. 

As for the BT-99: Great old single. They're great for trap, but you're out of luck in doubles, skeet, or sporting clays.  Browning still makes them, but by the time you've paid for an adjustable LOP, comb, and cast from Browning, you're up around $6K, and I think there are better options out there. As for Browning Citori O/U's - I've worked on a few of them where the inertia triggers didn't reset properly. I'm not a fan of inertia triggers, and much prefer mechanical triggers (ie, your pulling the trigger on the first barrel moves the trigger over to the second barrel, even if the first barrel doesn't fire). 


¹ There are some shotguns that have two beads on the rib - one in the front, one about mid-rib. These I find very slow to sight on a moving clay, but your mileage might vary. There are some shotguns used for "tactical" purposes that might have a ghost-ring type rear sight. These probably won't work well for trap and other clay games.