Your reasoning consists of he will turn 99 in to a bluff because I've seen it happen. Honestly people very rarely do this at 100NL not to mention this isn't a good spot for it seeing as once you check 99 should be ahead of most your range unless he can see your cards.
As for him doing this with nothing, what air can he hold? 79s if he's loose enough pre, maybe T9s if he's loose enough on the flop, there really isn't much. Most his draws have hit leaving us with little air within his range.
His value holding won't get much weaker than AQ and even that he never folds when he has a spade and may not fold even without a spade as he's pretty fishy. We don't even know if he floats it or value bets it so it's a fairly optimistic weakest value hand, when we remove AQ we're left trying to bluff a fishy player who probably has a flush off sets whilst setting him pretty good odds
The few times we run in to a straight draw or an insanely played AK will not make up for us shovelling money in to a very strong range.