[stigmata]

I would be tempted to call the river here. You only need to win 1/9, his aggression isn't really really low. His river aggression is tiny, but over 50 hands this is the street thats going to be more affected by the low number of hands. He may be passive, but if he is even slighly aware, he has absolutely no reason to think his A4 etc. hand is beaten, particularly if you have been playing very aggresively.

However, I really need to find more spots to fold on the river. Let's see what the others think....