No.

Even if his work were copyrighted, you still wouldn't have to give credit. There's two major principles involved in copyright/plagiarism law:

1. "Access" - The alleged plagiarizer must have accessed the material

and

2. "Similarity" - The "plagiarized" work must be identical or share components that are unique to the work in question (i.e., "nothing else like it")

Neither of these apply to you. Saying you committed plagiarism would be like saying everyone who wrote a love song has committed plagiarism against whoever the first person to write a love song was, or that turkey violates chicken's copyright on bird meat.

**EDIT: Oh yeah, almost forgot...

I am not currently a member of any bar association, and this should not be used as a substitute for legal help nor presented in any legal matter for any reason.