I found your grandfather in Rutherford County in 1910, by information from James Hoyt Holt's birth certificate information I got thru the familysearch.org's Search Records/Search Pilot. It states that "Hoyt" was born on 1-9-1912 in Rutherford County, and Bob was born in Cannon County, Mattie in Rutherford Co. What is strange is that there's another transcription of the information with Hoyt's names misspelled, the date of birth being 11-9-1911, Bob being born in MO, and Mattie in Cannon Co. It's likely that Mattie was born in Rutherford, as I found her in the 1900 census, born September 1889,there with her parents, William and Linday? Harrell. Robert's middle name is G. on their marriage information, and also in the 1910 census in Rutherford Co., TN in the 18th Civil District. It shows him as being 26, born MN?, 2nd marriage?, wife Mattie 20, TN first marriage, married 2 years, 0 children. This looks to be correct, as R. G. Holt married Mattie Harrell in Rutherford Co., TN on 11-17-1907.
Their ages in 1920: Bob 37, and Mattie 29, look like it could be the right ones.
They also had a son who died as a baby: Leroy Holt, born 5-7-1918 TN, died 10-14-1918, in Newsom, Davidson Co., TN, buried in the family cemetery. 1918 was the year of the Great Influenza Epidemic.
I might have found Bob in 1900 in Cannon Co., TN 7th District, as Robert Holt, 19 years old, farming with another family, single, born TN, and I tried to find his parents, but no luck so far.
You might try queries in Cannon County for Bob and Rutherford for both of them. I checked RootsWeb but found nothing on either of them. You can also try a query to the RootsWeb Message Boards. I did a little genealogy on some Harrells, but in Lauderdale Co., West TN.
Good Luck! I just like doing research sometimes- I have no family here.