BOF posted some great links that I also definitely recommend checking out. To answer the OP's question, it really all depends on the athlete. For athletes who are "tight" and generally have mobility issues, yoga is great. For athletes who have hypermobile joints and are "loose" yoga can actually be detrimental as it can increase the risk of injury.
To simplify it even more, most athletes either have a stability or mobility issue. Some are kind of right in the middle and can benefit from both, but most of the athletes I work with are either on one far side or the other. To determine what side you're on you can perform a simple joint laxity test. To view how to do the test go to this link here: https://www.youtube.com/watch?...CxjFL3Tk&index=4
To score this test:
Give yourself a 1 if you can make an angle of 90* or > between your pinky and top of your hand. Score this for both sides.
Give yourself a 1 if you can touch your thumb to your forearm. Score this for both sides.
Give yourself a 1 if you can make a 10* or > angle between your upper arm and forearm (moderate to significant elbow hyperextension). Score this for both sides.
Lastly, give yourself a 1 if you can touch both palms to the floor while keeping your legs straight.
Tally up your total score out of 7. If you score a 0-2 your joints are naturally "tight" and you can greatly benefit from yoga. if you score a 3-4 you are kind of right in the middle and we'd probably need to look at your joints individually in more detail to determine which joints need more mobility and what joints need more stability. If you score a 5-7 your joints are naturally "loose" and yoga might actually increase your risk for injury. We need to do more stability work with you and very little to no stretching.
Hope this post helps some of y'all!
Thank you to all that replied. This has been a huge help and I will pass on to son that he is most def not wasting his time. Above I can go ahead and tell you son probably has mobility issue. He is not just super flexible which is why he wanted to start in the first place I believe. Also, on a side note, son did a workout called Kempo today. Said it was super fun, lots of cardio, lots of core, and he was dripping sweat when he was finished. Anyone else worked out like this before?