We have had this topic come up before in this forum. Unfortunately, elevation masks seem to be the latest thing for some fitness/athletic groups, These masks restrict breathing and can be dangerous. I personally do not recommend them, but have seen them in the gym (primarily worn by youths seeking to improve their endurance). There are better ways to do this.
I would choose a safer method to increase endurance and have your nephew seek a reputable trainer or sports and conditioning coach for assistance in his sports. Others may have more information, and if there is any scientific evidence to support these masks I would be interested in seeing it.