Very interesting that he salivates when in a halter or bosal as well, I haven't seen that before!
Is he a relaxed type of horse, or quite anxious?
It could go either way - if he is super relaxed and enjoys being worked, he may be relaxing his jaw and 'chewing' which released saliva.
If he's worked up and tense about being ridden, he could be releasing saliva through stress.
It could also very well be just his little 'quirk'.