DP's dad is a very nice man. He helps us out a lot, and is very pleasant.
However there is one negative, and it's driving me a little bit insane.
When he comes to see us, he never puts the toilet seat down. This is a real bug bear of mine, and fortunately DP always put it down. I can just about (reluctantly) deal with DP's dad doing this, but the problem is there is always piss all over the seat (as in, under the seat you sit on and not in the bowl itself). It means I always have to clean up after he's been in there and it's just not pleasant.
I'm not sure what to do - if I mention it to DP he will bring it up with his dad and embarrass him and I don't want to cause anyone to feel uncomfortable. But at the same time, I don't know why I should have to clean this up 
it's really odd for DP's dad to behave this way as in all other ways he's considerate and a bit of a 'clean freak'. He is not old or infirm and there's no reason for this to happen, I just don't know why he doesn't clean up the mess :s What is the best course of action?