Angi, that is great that you have at least father and son, living with paternal grandparents in 1911. Where did Robert Jnr show as being born?
From your grandfather Robert Jnr.'s death:
1993 MCDOUGALL ROBERT JOHN ELDER ELDER M 89 DUMFRIES /DUMFRIES AND GALLOWAY
Have you viewed this? If young Robert's parents were not married when he was born (and things seems to be pointing that way), sometimes marriage or death certs may include mother's married surname (from later years) which at least can help trying to figure out the maternal side.
Monica