You should had asked him then and there for your knowledge that why he told you not to greet him, but since for reasons you refrained to do so hence i suggest you to just forget about it, people make mistakes and after knowing from you being non muslim, he should had replied Alaikum (upon you).

Clarifications
A long hadidth which is narrated from Aishah (ra) the mothers of the believer of an incident where a group of Jews visited prophet Muhammad (SAW) and said As-Samu alaika (death be on you) and Allah's messenger (SAW)replied Alaikum (upon you), recorded in Bukhari.

The hadidth is long with a detailed message to be kind and lenient in all matter, i can provide more details if some one seriously want to read arabic/english

Mr. Blazer, with due respect to your information above i suggest to consider when one is quoting prophet Muhammad (SAW) one should be sure of the incident and try to communciate exact words of the blessed prophet (SAW) or one should chose to be silent..

Allah knows best.

Peace to All :-)