phlegm attacks

my boyfriend is nearly 18 and doesnt smoke, he leads a healthy life style and takes part in alot of sports several times a week. about 3 years ago he had a serious chest infection which lead to him being hospitalised. he recovered well and had no further problems. about a year ago we went to a festival and he was anxious, he gagged and vomited clear phlegm. whenever we go anywhere with friends or somewhere new it all happens again. once hes done it he feels fine. hes been tested for asthma which came back clear, hes been given a nasal spray which didnt help. doctors claimed its a pshycalogical problem and its all in his head so he has visited a therapist which gave him breathing exesizes to help cope with the problem which just hold it back for a few minuets. its gone on too long with no help. any ideas on whats happening to him? any ideas on how to treat it?