My Husband Is 45 Years Old And Has Had 3

My husband is 45 years old and has had 3 episodes of fainting/black out (lasting a couple of minutes) in the past 3 months. Each time he was travelling on work (Korea, Singapore and US) and was taken to the hospital and checked and nothing seems to be wrong with him according to the tests. He is a very healthy man who exercises every day and looks very fit. His BP, blood sugar etc is normal. He has also had a full medical check up done and nothing has come up. It could be un related but in this period he also has been having very itchy hives come up off and on on his body specially in the evenings...these go away in the morning.What could be causing all this?