I think I'd suggest he make a cardio appointment just to be sure nothing is going on. When I asked my doctor about low BP, she said it's important to know if I am symptomatic (dizziness, lightheaded, shortness of breath). It is entirely possible that your son is in tip-top shape, but given all of the changes in his life, I think I'd want to double check.
Maybe your son is short of breath because of his pulse being so low. I know when i was on beta blockers my pulse was in the 30's and i could barely breath. I know you said he isn't on meds but the low pulse could very well be causing the s.o.b. symptoms. Maybe the Cardio could put him on something to raise his heart rate and see if he breathes better. Has he been checked for asthma? I know alot of athletes have a very low heart rate because there heart is so healthy but if its causing shortness of breath then maybe you should look into it. Or maybe it could be anxiety. Has he ever been DX with anxiety?
Hope he feels better soon!!
Blessings,
Tara :)