Andy, do you have a home BP monitor? I'd check him whenever he feels off. Something is causing acute hypotension, and the hypotension is apparently causing the blackouts. You need to try and detect some pattern.
It's likely to be a heart problem. But it might be a drug problem. I'd take his BP every two hours for a couple of days, and find out when his BP goes naturally low. BP usually follows a cycle, and not everybody is the same.