Hello Dorothy,
William Young. died WW1 1917
We would need to narrow down who William Young was in himself, before we have any chance of identifying him in any WW1 record/Index Cards. Do you know any more about him?
The best start is to have the timeline - sometimes there are identifying details on records that give clues to other records:
Mary Ann Cooper
Date & place of birth
Names of parents
Marriage 1) details ( names, places, dates?) What happened to husband 1 - divorce? Death?
Marriage 2) details ( names, places, dates?)
Marriage 3) details ( names, places, dates?)
You can mention the names of the children Mary Ann had to each marriage - but only if they have since passed.
As to whether or not William Young is your Dad's father - just because Dad was over 5 years old when his mother Mary Ann met William Young , doesn't necessarily mean he couldn't be the the father. More unlikely, yes perhaps - but can't categorically say no. Also, when you say "met" , do you mean "married" or is there more to a "met" story?"
( Have you ever thought to do DNA, yourself and your siblings? To try and works out some of the tangles in your tree

)
Cheers
AMBLY