Peeing in house

A few things in your post strike me. First, not all dogs understand that not peeing in their own house means not peeing in other buildings. Some are quicker to get that idea than others. One of the Rottie girls I have now was really slow about it and embarrassing when I took her anywhere for all too long. So what happened at the bf's parents' house isn't unusual. Since your guy isn't to the point of understanding "not indoors anywhere," if you take him someplace like that again, you have to treat him like an unhousebroken puppy. Him being more excited and liable to pee when company comes is also not uncommon.

Second, why are you letting him "just sit there" or "stare and walk and pee"? Leave a collar with a short grab line on it on him in the house, and when he starts that, grab him and get him outside before he's hardly started. If you can do something that startles him and makes him stop the flow long enough for you to do that, even better. If you have to drag him, do it. Slippery floors help.

Last, a 9-month-old and he's sitting while he pees? Do you really mean that or just that he's still squatting as opposed to leg lifting? And walking as he pees? If those are really accurate descriptions of what he's doing, I think I'd be asking the vet about it as it strikes me as unusual. Maybe people who have raised more males than I have will chime in. Is he able to hold it at night?

Okay, that sounds much more typical! It's very common for males to not start lifting their legs until they're more mature, and giant breeds are slow to mature to begin with, so the squatting isn't a concern. Thank you for the clarification, with that I'd say approach this as a potty training issue.

Guests are probably very exciting and/or interesting to him, and may distract him from paying attention to his bladder or make it harder for him to contain himself... literally. If he's wandering off to pee somewhere when the guests are over, consider tethering him to you (basically keeping him on a lead) or confining him to the room you're in so you can keep a close eye on him and get him out if he starts showing any kind of pre-potty behavior like sniffing, circling, pacing/restlessness. Plan on doing extra potty breaks during these times as well. Definitely be extra on top of going out with him and throwing an immediate post-pee reward party whenever there's guests in the house or are visiting someone else's house. Make sure you reward him for finishing outside even if he started in the house, to help motivate him to try to stop peeing and go outside with you when he's having accident. All this will help reinforce that following the potty rules are still super rewarding, even in new and challenging scenarios.