Please stay seated during entire performance, it's not as long as you think.
Well get him some rubber gloves and a mop and bucket. Some cleaning rags and a good general cleaning solution. Mr. Clean has some nice ones that have Frebreeze (sp) kills the smells. Make him clean the bathroom toilet, tub, sink, counter top, mirror and floor daily.
I hate shaming or embarressing kids but sometimes it's the only thing that works. If a close relative comes over and needs to use the bathroom. Say something like watch out for Tommy's puddles, try as he might he never seems to hit the toilet. Grandma, Grandpa aunts and uncles will laugh and maybe one of the males will take him aside and explain that if he ever wants to have a nice girlfriend and later a wife he needs to hit the target.
For me this is a deal breaker while dating. If I am dating a man and his bathroom smells like old pee, or the seat is wet, or up all the time. I will tell him how I feel but if he can't improve that about himself --- I'm gone.
I cleaned up after myself if I had to go to the bathroom and my tampon was overflowing and I dripped on the floor or seat. Who wants to use a bathroom with blood on the floor or seat? It's just good manners.