Suspected Dog Abuse?

I'm not sure what to do. I have been pet sitting for a little over a year now, and there is a dog I have gotten 3 times, and its usually always for a period of 3 weeks at a time.

He is a border collie/whippet mix supposedly, sweetest thing ever. However while I care for him, I noticed little things, like he cowers at loud bangs, if something falls off a table, or if my boyfriend yells at his video game. It's not directed towards him, but he immediately cowers under our table and shakes.

You can't touch his tail, or ANYWHERE near his backend/stomach area. He immediately jumps at the slightest touch, puts his ears back and tries to get away from me.

One time he accidentally knocked over a lamp, as I ran to try and catch it while it wobbled on the table, he ran from me with tail tucked, ears down and wouldn't come near me for the entire day even when I tried to entice him with treats.

However when things are calm in the house, he is a pure baby. He loves to cuddle and is completely glued to me. Follows me to the bathroom, kitchen, he immediately jumps on my lap to sleep when I sit down on the couch and he sleeps ontop of me at night, even though the owners told me hes not allowed on furniture.

I tried doing that silly test where you lift your hand as if you're going to hit your dog and see their reaction, and he fell to the floor the moment i lifted my hand.

When it's time for him to go home, he never wants to leave my side. I take him outside to meet his owners and he stays glued to my legs, and just looks completely scared. His owner has to drag him into his car.

However when he gets dropped off, his owner includes a lot of luxury things that I dont get with other dogs. He has a huge bed, top quality treats and food toppers, a bunch of chews that could last a month, brushes and expensive harnesses. My boyfriend tells me they wouldn't spend so much money on a dog they treat terribly.

I have asked the owner why he acts so scared, and he gives me a generic response of "Hes been like that since he was a puppy."

I absolutely adore this dog and I feel so guilty everytime its time to send him home. Should I do something? CAN I do something? Or am I overreacting?