We think it may be that he was brought up by maternal grandmother and took her surname. In which case we might never find out. One interesting thing in the Scottish records though that does make me wonder is that a "Janet Patrick Stringer" who died in 1950 in Glasgow - is that the grandmother and as he claims his father hated him did he just change Janet to John on the marriage certificate. Sadly, as far as I can tell Janet never had children and so another potential line of enquiry dies....
Just a few things Daisy...
Have you found Janet Patrick Stringer on any census records?
Do you have her death cert? May hold clues with names...who registered her death?
How do you know she had no children?
"Patrick" is also a known surname.
Mick/Mickey would be more known as an Irishman's name for Michael whereas a Scotsman would be more likely known as Mike/Mikey.....if this helps?
Mickey may also have been a referral to someone constantly joking/telling fables.....a thought?
What were the forename of any daughter's he had....could be another clue if you can discount any from his wife's line?
ADDED......Was Janet Patrick Stringer married/widowed when she died?
Could she have had a daughter under her maiden name but "Dominic" took on the surname of Janet's husband if she was his grandmother?
Annie