Shelter Dog loves to fetch but seeks to bury the ball instead of return with it

I'm taking care of my friends' 5-year old beautiful golden retriever. He has a major hoarding issue - at their house, where a doggy door allows him to go in and out as he pleases, he has a frustrating habit of taking their shoes and then burying them on their 1 acre property. Related to this behavior, he loves to chase a ball when thrown, but then he immediately runs around looking for a place to bury it. They don't spend any time training him, but I have him for 10 days and am committed to at least getting started. He normally comes when called, but not when he's looking for a place to bury something. Can anyone give me a suggestion for how to train him to return the ball? He needs a lot of exercise, and it would be nice to be able to use the Chuck-It to take care of some of that!
Thank you!

I realize some time has passed since you posted this, but I am willing to give you advice for when you have to watch this dog again, or to pass along to his owner.

Believe it or not, exercise comes from more than just throwing a ball around a backyard, especially with retrievers. I suggest a good 45 minute walk around the neighborhood (if possible) at least twice a day. Retrievers tend to have a "go switch," as I like to call it. Right when they wake up, they just need to go, go, go all the time, especially when someone is paying attention to them. On top of the walks, I suggest using a clicker or some treats and have that be a training exercise for him. Throw the ball, then right as you see him start to bury it, call him back with treats or the clicker. Redirecting is key to getting a dog to stop a certain behavior. Eventually, he will learn that he gets a treat when he brings the ball back and will do it more and more to where you won't have to use treats anymore. As for the shoes, you need to establish ownership. Not everything that is on the ground belongs to dogs. Make sure you redirect there as well.

Hello there,

One thing you could try is to use a toy with a treat inside.

For instance, take a tennis ball, make a cut on it and put a sausage inside.

Make sure your dog sees you putting the sausage in there and let him smell it.

Once he knows the sausage is in there play regular fetch, like here:
How to teach a dog to fetch
How to teach a dog to fetch - Part 2

Instead of trying to bury it he most likely will want to take the sausage out.

Once he retrives the ball to you, do just that, take the treat out of the ball and give to him.

I bet he will soon learn.

As a last tip, try training this indoors so that he doesn't have a place to bury the toy and the most interesting alternative for him will be to get the treat from the ball.

hope is helps ;)