There could very well be a direct connection Tom Tom. Is he using a face mask or a mouth piece? Face mask would allow his nose to get irritated by the HTS.
It could be dry air tho the HTS should help that & last, but not least is coughing or even holding in a cough....
So many things can be causing it. There was another thread about nose bleeds recently also. Another thought given his medical issues....does he happen to have high blood pressure for his age by any chance?
I guess I havent given you any real answers, but to let you know it doesnt surprise me. HTS is great, but it can be irritating!