Maybe his knowing that he is going to be ridden gets his adrenaline going? My percheron mare salivates hugely when she eats her grain....she ends up with soup. I'm thinking she gets so excited that it makes her salivate. She is full of saliva when she is ridden too. She does have a copper roller snaffle. She also drools when she is relaxed while you groom her and scratch her belly.
A horse can produce saliva depending on the bit (my horse has a bit with copper rollers and she does) if they have clover in their field or if while they are riding their hed is in the correct position (if it is they will)