At the track we teach horses to pee on command by whistling, we do this to get them to go when there is drug testing.....we would whistle everytime the horse would go and after a while they would associate whistling with peeing.....perhaps you could teach him to do this and it would empty him out before you ride?
Also, what kind of footing are you working in? Lots of geldings will find something to pee on/over to stop their legs getting splashed with urine....I've seen geldings pee on perfectly fine hay just to avoid getting their legs wet
If you are in footing that is deep and stops splashing he may be taken advantage of peeng on it? I've seen a gelding in a great big paddock walk all the way over to the hay to pee on it!
Just a thought?