Welcome. I have been watching and actually made an offer on your Patrick Nagel "Cowboy" on Ebay. It's very cool to have a proto-Nagel drawing and yours is unique. My best guess based on information from another avid Nagel collector is that it was done in or about 1977. This guess is based on at least two very similar pieces the Nagel world is aware of. "Divine" and "Pat Ast" (with the fan). Both of these images are in the book of celebrity illustration called "Fame." These are the two Nagel's in that book. Divine and Pat Ast, who was a model for Halston and appeared in Warhol movies.
So far that is what I have found. But there are a few other pieces around that time frame that resemble this early style. Your cowboy is very nice. Early works are harder to sell and the male images without proof of direct provenance might be tougher, but your Cowboy is lovely indeed. I love Nagel's male images, sometimes even more then his women.
Hope this helps some.