The 3 big resorts in Summit County UT do half the visits of the 3 big resorts in Summit County CO, so I see no issues there.
Your son's spring break week is the same as my timeshare week at Snowbird. I only see major lineups on big powder days. If it's warm in March the lower elevations around Park City can get sloppy. They also have a lot of flats, though not as traverse-intensive as Breck. These are potential issues for your son learning snowboarding. Snowbird is not a good place for a beginner at anything, but Brighton/Solitude might be worthwhile if it's slushy at Park City. You should stay in Salt Lake with your son to save $$$, particularly since it's 45 minutes max drive to Park City and in March you probably won;t be skiing those areas more than half the time anyway. And you can get discount tix to the PC areas in Salt Lake but not in PC. If your son switches back to skiing you should go to Alta for a day also.
This is a very good idea. The 2011 Gathering at Breck was in that timeframe and I saw few crowd issues and snow on the upper half of the mountain was excellent. For 2015 one caveat is that Easter is April 5. The prior week is very busy at Vail, not sure how much Breck is impacted.