Perhaps switch rooms - so his room is not next to the restroom, if at all possible...

when nature calls > convenience of a selfish roommate

Tell Sheldon Cooper that this was NOT part of your roommate agreement and you do NOT need to adhere to a bathroom schedule. Flush that toilet and yell Bazinga!

Having had many wacky housemates, I was very curious about the community response. Making a request that people flush as little as possible after 10pm, if asked respectfully...sure, reasonable. Unreasonable+power dynamics tend to get unworkable, quickly. Start documenting these conversations/requests. You may need it in the future. There are silicon earplugs called "Mack's" that can be found at any pharmacy section. They are incredible. I can sleep through anything with them, and I am a light sleeper too. Good luck!

Tell your flatmate to either deal with the flushing or find some place else to live. That is an asinine Request.

Being a light sleeper is a personal problem. Fecal coliforms affect everyone.

Sounds like he's full of almost as much shit as your toilet

Tell him not to go to sleep past 10 because you guys use the toilet.

There's a Mitchell and Webb Look skit about this, which recommends not flushing at night unless it's a poo, in which case you yell out, "IT WAS A POO! IT WAS A POO!" I suggest adapting this measure.

Is it not common decency to wait until the next morning? In my house we have the rule 'if it's yellow let it mellow, if it's brown flush it down' during the night. During the day flush as much as you want but at night it's just courteous to wait to flush pee until the morning. At least that's how I was brought up