[goofball]

I think this is almost a fold but not quite. If you're opponent is a good second level thinker and knows you probably have an A and knows you'll bet it then he'll checkraise because he has a set. It's stretching to say he knows you know this and will expect you to fold. However, he could be doing it for value with a worse hand or he could just think you're full of [censored]. You mentioned "you thought he was pretty passive" Change 'thoguht' to 'know' and 'pretty' to 'very' and it's a fold ez.