Regeneration has little to do with countering the kind of poison Sasori likely used. Regeneration just makes the body replace lost tissue. Poisons either affect the tissue locally (like damaged mucle or whatever part was struck) or flow through your vascular system affecting some specific target, hampering the normal functions of the body. The first case you could in theory regenerate but I don't think Sasori's poison was of that sort.

And you are only looking at one side of the coin, anyway. Sasori is, or was, quite a villain himself. I don't think he would have come unprepared. Even if he assumed Kabuto would still be loyal, in general you could say he wouldn't have probably gained a position in Akatsuki by being a fool who can't see other possibilities and be ready for any surprises. So, just in case, if he had thought Oro would have known all about his poison, he would have switched to another one for the duration of this meeting.