First, I think its awesome that your nephew is skiing. I credit much of my success in fighting CF to my year-round high level of physical activity, and coming from New England, being able to enjoy winter and participating in winter sports makes a huge difference.
Personally, I've been ok with altitude, but find that I have a lot more energy skiing at lower levels. Its a trade off... the snow out west, enjoy the cruisers and the intermediates, or do more challenging stuff on smaller mountains. I've been skiing as high as 14,000 (Arapahoe Basin, CO). Yes, I've had some fatigue issues, and would recommend taking several days to acclimate if you are coming from sea level. I have had several conversations with my Dr's about this, and would encourage you to do the same. I was told that it should not be detrimental unless one has seriously compromised lung function. In my experience, if he is doing well enough to ski, he should be able to handle it. He may fatigue very easily at higher altitude, but shouldn't suffer long term problems... he may enjoy skiing at lower levels more. Let him do what he can handle, and be ready to take it easy if that's what he needs.
If you go up into high altitude, HYDRATE... otherwise, altitude sickness/ fatigue will be an issue.
Here is an example of slowing building up to higher altitudes from a trip I took last year.
1) Fly from sea level to denver (@5,000 ft)
2) Stay in Denver for about 36 hours.
3) Drive up a little higher... we spent another day in Keystone/ Silverthorn before skiing, which added another few thousand ft.
4) 2 days of skiing at lower mountains .... Keystone one day, breckenridge the next.... breck has one very high peak, which i avoided.
5) On the third day of skiing, (5th day of trip) I was ready to go higher.... went to A-Basin, which tops out at around 14,000 ft.
6) Next day rested (very jealous, as all of my skiing buddies went to Copper Mtn. that day) <img src="i/expressions/face-icon-small-sad.gif" border="0">
7) Last big Ski day... Vail.
8) Drove back to Denver
9) Fly back east to the land of Ice and rain :-(
One last word of advice... Free the heal, and Free the Mind!!!! http://www.ustsa.org/
Chris
25 w/ CF