It is a common occurence. If this is his first time I would be highly upset that the doctor isnt more concerned, but at least you contacted him.
Those of us who have long histories have pretty much learned to tell when there is a real problems versus a trigger that started the bleed. For me anything from PMS, excess caffeine or fatigue & straining to life something to an actual infection can trigger it.
For now I would suggest holding off on Pulmozyme if he uses it & any CPT he does until he is blood free for 24 hours. This is a typical response for a bleed, but if you arent comfortable with it run it past his doctor.
The extra Vitamin K is good. Ice water to drink & if was bad enough (you feel scared enough) ice packs on his chest. He really should NOT lay flat or over do anything until he is blood free.
IF the bleeds continue (fresh blood, not the old stuff that he might still cough up) than I would contact his doctor again. If you are just not sure then be a pain with the doctor.
Good Luck & Let us know what happens!