How to diminish separation anxiety for a dog that isn't mine?

I just moved back home after college and I've been walking my neighbor's dog. Let me just say that while the dog is cared for with proper food and water, they just don't give it the attention I know it requires. I've always wanted a dog and this is a good way to get some experience and get a workout.

The dog is a St. Bernard and he's an absolute gentle giant. My neighbors bought it as a puppy and have had it for 4 years. They decided to have kids and they've dedicated the last 3 years to having 2 children. While there's nothing wrong with that, they just don't walk or interact with the dog very much. They're either working or tending to the kids. The only one willing to take him on walks is the dad but he comes home from work tired. They have lost interest in it.

My neighbors refuse to give him up to the pound and while I would love to keep him, my parents just don't want a dog and I have to respect that.

I have walked the dog everyday the past week and he's very excited to see me everytime. Today, I didn't have time to walk him and I've been hearing him cry and bark for the last five hours. I'm sure it's because he didn't go on a walk today.

While I do feel for the dog because the backyard is very small and there are days that they don't interact with him at all, I just can't tend to him everyday.

I'm sure the excitement of a whole week of fun walks followed by the return to solitude even for a day affects him.

Is there anything I can do to have him more relaxed on the days I don't walk him?

The owner had to come out and quiet him a bit because he kept scratching at the gate and barking. He's usually quiet when he isn't interacted with. I know it's not my responsibility, but I feel a little responsible for this behavior. I have 0 dog experience and because he's not my dog, there's only so much I can do.