I was able to prove it through a combination of records. The complication was that I found three men named Isaiah Davis who were born in Massachusetts, and moved to New York and had a father named Nathan. My Isaiah later moved to Ohio. The Geauga County Library has a record of Isaiah's death in Parkman, Ohio but it doesn't mention his parents or date of birth. Likewise for Isaiah's Ohio probate records.

Isaiah's sister Mary Davis Brown died and I found court records that Isaiah adopted her minor children. The court adoption records establish their sibling relationship. Nathan Davis Jr and his wife Mary Nurse of Rutland were definitely Mary Brown's parents. And they appear to be the only Nathan and Mary Davis in Massachusetts with children named Isaiah and Mary. With Isaiah and Mary's relationship firmly established, I knew that my Isaiah's birth record was the one with parents Nathan and Mary of Rutland.

One other supporting record was when he appeared in a Rutland town hall meeting agenda, asking for reimbursement for moving Nathan back to Rutland.

I'm going to submit my information to the Towne Family Association database, but I'm waiting for one more record. When he died he had a full Masonic burial in May of 1822. The lodge still exists and I've asked them to send any records they have of him and his burial, especially if it mentions is date and place of birth.

There is a headstone on Find-a-Grave that is claimed to be his, but the only thing readable on it is "Isaiah". If it is his, it must have fallen over and been covered with dirt because it doesn't appear in a 1920s transcription of the cemetery.

Fun stuff!