I don't recall the details but look up something like "sleep paralyses" I believe we all enter a paralyses state when we sleep or at least when we dream - this is the body's way to protect itself form violent dream caused reactions.
It is rather normal for people not to remember dreams.. so I think it is possible you father was having a nightmare and was calling out (seems that paralyses doesn't include the loss of speach), and when you tried to wake him he was in a delayed paralyses state for that period of time it took you to bring him "around".
It think there must be some thread on thiis subject on Med Help.
In fact here is one area on that subject:
http://www.medhelp.org/search?utf8=%26%23x2713%3B&query=sleep+paralyses&camp=top_nav_search