Hi Robert
Welcome to RootsChat
Really hard this type of search, as you have found sadly over the years of your research. As you will know, there was no paper trail for the unofficial and private adoptions that took place around this time (with adoption not becoming a legal process in Scotland until 1930).
From what you have and I have seen elsewhere:
Birth Certificate shows a Barbara Ross, Hotel Tablemaid, domicile - High Street - Invergordon as his mother, no father provided on certificate.Barbara Ross may have went down to Glasgow to have the baby, your father. Or, she was a Glasgow girl, who happened to be working up north when she became pregnant and then came home to have her baby.
The family that raised him, did they marry in Hamilton Lanarkshire in 1907 (just read some very old posts elsewhere which mentioned Robina's maiden name as Murdoch)? A marriage in 1907 between an Alexander Currie and a Robina Murdoch in Hamilton and thought this might be the couple who were your grandparents? Just a guess from me.
If so, curious about the name given to your father in 1908 by Barbara Ross of Alexander Currie Ross and whether there was any earlier connection between Alexander Currie and Barbara Ross. Have you followed up the genealogy of the couple that adopted your father? I can see more on Robina Cunningham Murdoch's family (although tricky to find them on censuses overall).
Did Robina Murdoch remarry?
www.findagrave.com/cgi-bin/fg.cgi?page=gr&GRid=109995132These searches are so hard, so trying to see if there are any other sources of info that might provide you with further clues. Sometimes, fingers crossed, going sideways can help with new info....when things are really hard
Have you found your father on the 1911 census?
Monica