It's not shooting a proper fitting stock that causes a young (or any) shooter to shift back. It's natural for a new shooter to try to assume a position where his combined center of gravity is between his feet. Most are familiar with the back lean of the off-hand rifle competitor. This may work well for fixed targets, but not for wing shooting.
Having a youth use a too short stock will shift the gun's center of gravity and possibly reduce the back lean. IMHO, a better solution is to have a proper stock fit and shift the C of G by shortening the barrel or shifting weight. That's why many youth guns have short stocks and short barrels. Just whacking some wood off of a long barreled adult gun's stock will foster a back lean. Many young shooters can handle a heavy gun if the balance is correct -- It won't take them long to develop the wing shooters stance with a proper weight distribution. But, if the C of G too far forward don't be surprised to see a compensating back lean. When I signed-in future Olympic and International champion Kimberly Rhode at one of her first NSSA tournaments, she was still a little girl (11, as I recall) but she had no trouble handling a 12-ga O/U (a properly fit Perazzi).
Yes, kids grow. I'd no more ask a young shooter to use a too short stock than I'd ask him to wear shoes than no longer fit. It's not that big an effort to add spacers as the young shooter grows. Many youth gun stocks (or recoil pads) come with an assortment of spacers. If I'm overly sensitive about too short stocks on youth guns it's because I got a broken nose because of one -- please, don't let the same thing happen to your young shooter.
If the OP gets an adult BPS with a short slug barrel and swaps the stock for a shortened one (with an assortment of spacers), it could be proper fitting and well balanced youth gun. Eventually, longer barrels and the original stock will give him a gun that will serve him well for a lifetime.
