It is unknown who the father of Samuel Douglas Ewbank was, because as you rightly say his birth and marriage cert do not give his father.
His mother was Sarah Ewbank who was born c1843 on the Isle of Man and she married John Weldon in 1873. Samuel Douglas Ewbank was born 14 January 1873 in Leigh, Lancashire and I believe was given up for adoption. I believe his adoptive mother was a Rachel Thompson and he assumed the name Thompson until his marriage to Elizabeth Waywell. Sarah married John Weldon in the September of 1873. John and Sarah went on to have 2 daughters, Maggie in 1878 and Maria in 1879.
Sarah's father was a William Ewbank and her mother was Charlotte Wattleworth - they married in Liverpool. William Ewbank's father was a Samuel Ewbank and he came from Kirkby Stephen. He married an Eleanor Bailey in Kirkby Stephen in 1807. Samuel was born in 1775 but I have been unable to identify who his father was. I have extensive information regarding the Ewbanks of Kirkby Stephen but I am not sure where Samuel fits in.
Regards
Julie Cartledge
nee Eubank