I found him in a Genealogy of the family of Forbes, from the account of Mr Mathew Lumsden of Tulliekerne, written in 1580 but published in 1819.
No sources and no dates and Mr Lumsden sounds like a disgruntled old man but he does have on page 50, Mr. George Forbes of Byth (not Blyth) who is son of Mr Wm, Forbes and Margaret Rutherfoord (probably old spelling for Rutherfurd) who married Bannerman (this is presumably her surname), daughter of the Laird of Wattertoune, and had one daughter who married Al. (Alexander?) Forbes of Gollan.
No mention of going to Finland or anywhere else in Scandinavia, or of having a son there, so that part of it is possibly invented?