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.
This subject has been covered in the past, so I will keep it as simple as possible. This device is not for everyone. Personally I know a few individuals who do use these masks for their training, but they don’t have the same effects for all them. I agree with Christine that there are better ways for him to increase his aerobic (and anaerobic) capacity. Hiring a fitness professional to design a constructive and effective training plan for him might be the best choice here.
The use of such masks is to create the effect of training at altitude. But there is a serious flaw in the premise that using a restricted oxygen supply will create better training adaptations. The main adaptation to training at altitude is that the number of red blood cells increase to assist in carrying more oxygen. But this is actually caused by living at the higher altitude for a long period. If you went up to a high altitude to train, but came right back down for the rest of the time, there would be no substantial adaptation. Your system would not adapt as the stimulus (low oxygen content of the air) would not be maintained long enough to cause the higher red blood cell production. Living at a higher altitude will increase your RBC count whether you exercise or not. And coming down to sea level will result in RBC count dropping down to normal levels in about 2 weeks even if you live at elevation normally. And there has not been nearly enough research into potential side effects of such training for anyone to just play around with the idea.
I see this as an attempt to create a short cut to better performance. But short cuts tend to have big downsides. Short cuts also tend to be short lived.