Your Grandma needs to have company over more often so Makis can get used to everyday noises. Poor baby. He sounds feral, but that isn't necessarily the case. Lil A was feral but since coming to live with me, she's still scared and will run and hide when someone comes over, but she's come so far since then.

Feliway is a good product. They sell an herbal calmer that I use on my cats. Just a couple of drops in water will do it.

Tell Grandma to lay off scaring him. It'll only make things worse.