Name of “John Oliver”, son of John and Margaret (nee Ball) Oliver, from, Katia, wife of Tim Grant Oliver, in an email dated 29 August 2021, located a burial document that lists a “John Oliver”, son of John and Margaret Oliver who was buried on 3 June 1763. In fact, this works out being three months earlier than his own mother, listed in the same page. (login required):
https://www.familysearch.org/ark:/61903/3:1:S3HY-6...%2F1%3A1%3AQGNZ-74PX
Register of Ashwater for the Year 1763 Delivered into Court at his Lordship's Visitation held at Okehampton June 5th 1764.
"John Son of John & Margaret Oliver was buried March 4th"

(Tim Oliver is a x3 great grandson of James and Ruth Oliver, his grandfather being Ivor James Clark Oliver 1907-1989).

Further searching found a christening record for this son John. He was christened on 8 Oct 1761 in Ashwater, making him aged about 2 1/2 years when he died:
https://www.familysearch.org/ark:/61903/1:1:N5QS-1Y9
https://www.familysearch.org/ark:/61903/1:1:N5QS-1Y9
Name: John Oliver
Sex: Male
Death Date: 4 Mar 1763
Christening Date: 8 Oct 1761
Christening Place: Ashwater, Devon, England, United Kingdom
Christening Place (Original): Ashwater, Devon, England
Father's Name: John Oliver
Father's Sex: Male
Mother's Name: Margaret
Mother's Sex: Female