Thank God, at this age it is not a sexual thing, just a biological thing. I only have one son who is 6 also, and I've told him that it was full of pee in the morning. He goes to the bathroom first thing when he wakes up, so it doesn't last long. I just tell him to leave it alone and it will go back to normal. I've never made a big deal about it. It was an issue, though, when he would pee straight through the middle of the toilet and it would go onto the wall and floor, but he's figured out how to aim it better now. It's okay for your son to want privacy, especially from his sisters. Tell him it's okay for him to shut the bathroom door when he's using the bathroom and to shut his bedroom door when he's getting dressed. Be sensitive to his feelings and just reassure him that it just happens and he shouldn't feel bad about it, but he still has to get dressed and ready in the morning. It always helps to make something seem more fun, too. Like timing how long it takes for him to get dressed and to the kitchen in the morning. Maybe buy a stopwatch. He could try to get faster and faster. That could distract him long enough for him to forget about the original problem and become used to it being normal. Like I always say, distractions are a mom's best friend (or at least the right ones can be). Do you think he adopted that new issue as an excuse not to get ready for school? I don't know if he likes school or not, but maybe that's the issue that needs to be addressed before he finds more excuses to be uncooperative in the mornings. If that might be part of it, maybe he just needs an encouraging talk about school and learning and how awesome it is.
He will get used to it happening, and it'll just become normal to him. From all the advice here, I'm sure you'll come up with something that works for you guys. Who knows, maybe by then it won't even be an issue anymore!