Persistence pays off. I called and left a message with the sleep center today, and, guess what? They had the results from Mason's sleep study.
Mason's pulmonologist called me with the preliminary results.  During the study, his average O2 sat was 96% (dipped to 93%); average heart rate was 65 (dipped to 50's) but was "regular"; breathed an average of 12 times/minutes; had episodes of snoring. Normal score is under 2. Mason scored a 3, which means only mild sleep apnea. This is good news - in that it's not severe, but it's something we'll continue to watch as he gets older. At this time the treatment would be worse than the symptoms. So, no CPAP, etc. We'll restart oxygen at night IF he needs it (but right now he's doing fine, so we're not even monitoring him).
We also discussed the persistent decreased endurance and labored breathing. Over the next few days/weeks we'll do some "trial" at home things. The first thing we'll do is take his pulse before exercise and after hard exercise. We'll look to see if his heart rate increases to around 100 (as it should). On another time, we'll pre-treat him with albuterol before hard exercise and see how he does.
On another note, today was his last day of Kindergarten! He's now officially moving to 1st grade and his new teacher will be Ms. Hallock...which I'm ecstatic with!