I know what you mean, except my son shares our bathroom. I make him stop what he's doing and come back to flush every.single.time. "I don't like to see a pee toilet... that's not my pee. So go right now and take care of it please."
You could either use a carrot, a stick, or a radical action.
Carrot: Make it sort of fun. "We have a problem with you forgetting to flush your pee, and our bathroom gets stinky. Each day, I'm going to go into your bathroom two or three times, and I won't tell you when. If the toilet is flushed, then you have earned a (dot, star, whatever mark) on this paper here. When you have fifty (that's about 2 weeks-- you want it to be long enough to change the habit so flushing becomes automatic) then you can choose (an outing/family activity...not a toy or possession). " We use a chart like this with our son for behavior/attitude and it works well. This will help him WANT to flush because it's in his best interest.
Stick: Cleaning up the bathroom each and every time you find pee, but only check once a day.
Radical action: take away his 'own' bathroom privileges. I'd try this as a last effort "wow, you really are having a hard time with this and we don't want a stinky house, so we need to close your bathroom until you can remember to take care of it."
Creative option: "Honey, you are needing help to remember to flush. What sort of thing can you make to help yourself remember? A sign?" He might have some ideas.
I'd be grossed out, too.