If you obtain the 1838 marriage certificate, it should give his father's name if known. The 1851 and 1861 censuses gave his place of birth as Oakford. A Thomas GREENSLADE, base child of Mary GREENSLADE, a servant, was baptised at Oakford 11 July 1813, and this seems to match a statement made on the marriage certificate. It is possible that PEARCE was the unknown father's surname.