Blaming yourself for not researching isn't going to do any good now, so try to let it go.
Sometimes vets just don't remember to mention things for whatever reason. I have read about a lot of people asking their vets about glucosamine after the diagnosis and the vet agrees but doesn't bring it up initially. If you are happy with your vet otherwise, I think I'd let this one slide and tell him that you will be using Cosequin...