It is worth checking for a will and also to see if the newspapers mention him at the time of his death. Also worth checking to see if there is a record of him graduating from one of the Scottish universities. Graduation lists sometimes give genealogical details.
My copy of the SND says of "Relief":
Freedom from ecclesiastical oppression, esp. applied to the controversy in the Church of Scotland led by Thomas Gillespie to assert the right of a congregation to elect its own minister. This led to a secession from the Establishment in 1761 and the formation of the Relief Church, an ecclesiastical body pledged to oppose patronage. In 1847, it amalgamated with the United Secession to form the United Presbyterian Church.
There were 115 Relief Church congregations when it merged with the United Secession in 1847.
