What kind of medical conditions? Are they things that people are typically biased against?

Why not tell him? Otherwise he may think that HE'S the reason why you're acting weird. He doesn't need much information since you guys are in the beginning of a relationship/friendship. A simple "hey, sorry if I've seemed out of it lately. I've been recently diagnosed with <insert medical conditions here> and haven't been feeling like myself."

He'll either shrug, (a signal that you should drop it), or he'll ask follow up questions. Be careful to be sensitive to how he's feeling and not to give too much information or to go on too long though.

I'm slightly concerned with his attitudes about your drinking. Was he feeling guilty because you didn't have a drink of any kind or was he pretty specific in trying to get alcohol into you?