4 year old boy ALWAYS pees on the floor... How to make it stop?

He is my nephew and we (myself, my boyfriend, sister in law, brother in law & the nephew) live together, his parents know about this problem they dont do anything about this... almost every single time this boy goes pee he completely misses the toilet and leaves a huge puddle of piss on the floor. He has been potty trained for over a year, so he KNOWS where he is supposed to pee. Even when he is told that he is not to pee on the floor he just laughs & says he knows. I find it EXTREMELY gross to go into the bathroom & find drying up pee on the floor...his parents just seem to not care, the kid doesnt care....what can i do? i wont clean it, its is not my kids mess to clean?? im almost gonna have my baby (im 38 weeks) & i just dont want to deal with this crap, it is nasty! Any advise?