Well, in a way I think he was because it was never contradicted in anyway in the show and I think they would've said it if they meant it to be a fake claim.

But my point of view is strictly in-story-world...ly:

The man's knowledge of the date and time Albert was abandoned at the orphanage's doorstep was the first evidence judge Adams pointed out and thus made it come off as their strongest evidence of the relation. And obviously something like that proves nothing about blood relations. He did say that the pile of papers provided more to prove it but apparently they had no DNA samples or anything that definite or else that would've been what judge Adams would've mentioned. And if I understood correctly Albert was 7 or 8 years old when he was abandoned.

So I was left with a feeling that they didn't have any absolute proof of the blood relation. And I can't figure out in my head why would a father who only wants a strong worker, abandon a 7 or 8-year old boy when he could've easily had him work in some chores already and MADE him strong as that was all he wanted.

So I'm thinking Jeremy Quinn was not the father but just someone who knew enough about the boy, perhaps had some relationship with the boy's family, and wanted him as a working man now that he saw a chance to get one for free.