I wish I could answer that, I know how hard it is to search for an unknown grandparent. Finding birth records in upstate NY before 1881 one can be difficult. Some towns have older birth records since you have a DOB to at least search around worth reaching out there to see. Church, cemetery, similar records might hold something.
I found a reference to the adoption here in an older genealogy where Simon and Louisa adopted 2 children:
"100 - Simon Harrington, 1828-1887, son of Richard Harrington and Deborah Milks, m. Louisa Pitts, 1844-1905. He operated the farm of his father, Richard, on Harrington Hill. Simon and Louisa are buried in the North Cambridge Cemetery. They adopted two children: 275* Horace Harrington, 1865 and 276* Grace Felt Harrington, 1878."(
https://www.seekingmyroots.com/members/files/G003068.pdf - p. 29 (23).
Unfortunately, it doesn't mention his biological parents but possibly there's something or someone in all of that might know more and has passed that information down. Easton is a small town in a very rural county. The person you're seeking could be from a long standing family in that area or not even close, you never know.
No DNA leads? I had no idea who one of my Grandfathers was when I starting searching for him several years ago. I had nothing but DNA to go on. It took quite a while several DNA tests and a lot of matches and trees to sort through but I eventually found him. Don't give up, lose hope.
