Have any other things changed? Our GSD is very aware of aspects we don't think to notice. She has always barked at men more than women, and that includes teen boys and not teen girls. All boy kiddos are cool, but once their voice deepens and they get taller, she is more cautious around them. She also HEARS things and alerts to those sounds. She can identify if my son's car pulls up in the driveway (without seeing it) or if a stranger's does. Or my husband's when he comes home at odd hours. Eventually she even learned the neighbors vehicles, and it takes a few days whenever our newspaper delivery person changes...
But, she ALWAYS barks at the trash/recycle trucks. They are big, noisy, and threatening. We've been here 11 years... she still barks at those people who take our things. LOL
She also has started to sometimes bark at "normal" things. She is getting old, and I suspect diminished vision and hearing are the cause. She'll be 13 next month. Certain sound ranges she can't hear as well now, I do know that. Maybe your dog has had a change in health status that is minor enough you wouldn't notice, but it affects her identification of normal sounds to some degree?
And if you're in a new home, there are likely a lot of new sounds to adjust to and learn. I think the idea of having her sleep with your son (or in his room) is a good one. If the dog will do it. Do you sleep with your bedroom door shut at night? If so, perhaps leaving it open or cracked so she can explore the sound without going nuts? Have your son offer her a treat first thing in the morning when he comes out, so she is happy and looking for him, rather than startled by his sudden appearance?
Mostly, routine is going to be the key. If your son's schedules is just chaotic with no rhyme or reason, it's not going to be easy to solve. I assume she stops her barking when she realizes who it is? (by then the damage is done, I get it).
As for the owner and his family, familiarity over time will help. Introduce him to the owner and the son, and encourage some gentle, calm interactions. Of course, if the dog gets a bad vibe from either of them, that may not go away ever.