I suspect the protagonist has dementia alongside complex PTSD, so it is not the first day he came home, it's been months or even years. I suspect that the daughter in law has left Arthur and is actually leaving with Timmy, not moving in (hence the boxes). The terrible reason he can't get into Arthur's room is that he just couldn't cope anymore and his body is slumped against the door. This is actually a fantastic depiction of how dementia can feel and the devastating impact it can have on the carer. I might be wrong but that's what it seems like.
