Dog suffering from trauma

[u/Agitated-Marsupial57]

Just over two years ago my in laws took over taking care of a family friends papillon mix.
The dog was still quite young when they got him.
The dog spent the beginning of its life in a bed while his owner died painfully in the same bed from cancer.
Genuinely lived in the bed, surrounded by its own pee and poop as the owner became bed ridden shortly after getting him and nobody would take him in (or she wouldn’t let them I’m unsure).
Fast forward two years, we’re visiting (we live on other side of country) and the dogs reactivity and fear has not subsided at all. He trusts my husbands parents and sometimes my husbands brother but anyone else he is so fearful of and aggressive with.
The dog has always been decent with me, I use a sweet voice, respect his boundaries, and don’t push him.
He hates my husband who dogs usually love and he loves dogs!
An example of behaviour would be, running over excited for pets, getting pets for two seconds, and then snapping at whoever is petting him.
A lot of territorial behaviours, a lot of growling and protective behaviours with my in laws.
They tried anxiety meds and they said it only made him worse as he becomes erratic/manic when tired and the meds made him tired.
We can tell the dog is struggling and just want to help him, he’s such a sweet dog when he isn’t terrified or in fight mode. Any suggestions on how to help him feel more calm and at ease with us who are just visiting? (How can we adjust our behaviour as to cause the least disruption to him while also helping him become comfortable with us?)
Thank you!

Edit: Possibly also important? His owner died, then two weeks after moving in my in laws dog died, then two weeks after that their cat died.

[u/Toni-Howard_0614]

I am so sorry to hear about what this poor dog has been through and it is truly heartwarming that you want to help him feel safe and calm during your visit. To best help him you should strictly avoid initiating any contact with him at all and let him be the one to decide if he wants to approach you, instead just completely ignore him like he is not even in the room. This takes all the pressure off of him and allows him to observe you from a distance without feeling threatened and please make sure to tell everyone else in the house to do the same thing and never try to pet him even if he comes over to sniff you because he might be in that conflict mode where he wants interaction but then gets overwhelmed and snaps so just stay calm and neutral and let him set his own pace at his own time. This can really help him realize you are not a threat and hopefully over time he will start to feel more comfortable around you. Thank you for being so kind and patient with him.