Poor Dexter seems depressed. He lacks confidence and is scared of many things.
They told me he had Cocker Spaniel and Sharpei in him but I would guess Staffie and Labrador. He has some characteristics I’ve seen in Staffies.
Depressed and shut down
It’s like Dexter is depressed and a bit shut down. A three-year-old should be more enthusiastic. He has had big changes in his life having moved to live with the couple from the young lady’s mothers house. Then he had the company of a GSD that sadly died.
Very likely the company of the other dog gave him reassurance.
In building his confidence I suggest they look at the overall picture and do a number of things a bit differently.
They will help him more with the things that make him anxious, like accompanying him out into the garden. At the same time they will give his life more enrichment.
The value of food
Most especially they will cut down on his food – particularly non-nutritious chews! How can a dog, having eaten three Dentastix (full of rubbish) have an appetite for his nourishing meals?
With food now having more value and Dexter just a little bit more hungry, food will become a useful tool for helping him to gain confidence. This should help make him less depressed.

Uneasy having his photo taken
I would also do walks a little differently. As depressed dog or one lacking in confidence needs to feel that his humans don’t lack confidence as well. How they react on seeing a person or dog is key, as any tension on their part will immediately transfer to Dexter.
They have a list of things to put in place over the next two or three weeks – some simple and some a bit harder. Let’s see how it goes. Added together they should help him to become less depressed and a bit more enthusiastic for life.