Stop drinking L O N G before bed. Go to the bathroom a few times before you go to sleep and have your mom wake you once or twice at night, under the guise of needing to speak with you privately. No one will know. Practice waking yourself up several times a night (use an alarm clock) for as many weeks as you have to practice to get your body used to waking for the bathroom. In a pinch, change to pj’s when everyone else does, and be the last to fall asleep, so that you can put on protective undergarments (pull up, they have them in larger sizes) when you’re the only one awake and no one will be the wiser.
A sleep over also does not have to be the only way to plan a bar mitzva. I had a sleep over/kiddush for my bas mitzvah, but no one else I knew did, or even does for their simchas.