I personally think the industry as a whole needs to take a step back and put more time/energy/knowledge into everything we do.
In general we tend to go to the needle or "quick fix" way to fast.
Its very hard to treat something correctly if you don't know WHAT you are treating. Stifles are very tricky to diagnose, He is awlful young to start injecting, just because. If he is more of the lighter/taller QH's he may just have a weak stifle joint, the best treatment for that is slow hillwork to help build that joints strength. Another possible injury is the cruciate ligament, that should be treated completely different, time off and slow flat work during recovery. A poor fitting saddle or any spine shift, needs to be addressed by a chiropractor. And improper shoeing can change his body weight,foot flight pattern and balance, this can be rectified by your blacksmith over a few months.
Without seeing the horse those are a few diiferent things that can be taking place. There are many others, so no, You really need a proper diagnosis to treat him correctly.
As far as injecting- I use that only on horses past their late teens and only as the very LAST resort. In the last 15 years probably have had 20 horses and only injected 1 hock as a last resort horse was 17 and an ottb, and belonged to a client. So in the end it was their decision.