You can try having two balls/items. When he is on the return run, wave the other item to encourage him to come. Swap toys and so forth.

That said, why must it be fetch? Despite the stereotype, some dogs just aren't into it. They rather play something else, like keep away, pouncing, or tug.

 

My dog refused to return tossed toys. She thought it was cooler to own it instead of playing with me.

I solved this with a combination of the two toys game and shaping. Fortunately my dog liked chasing the toy as much as she likes chewing and having it.

A support behavior: the dog knows hand touch or chin to hand targeting.

You'll need at least two identical toys. Toss one. When the dog grabs it, call him and entice him into chasing the other toy. When he goes after the second toy, pick up the first toy. Now repeat.

The new toss is the reward. At least it was for my dog. I shaped the return to hand by marking and tossing when she had just grabbed the toy. Now she learned that I might have some role in the game. Then I wanted her to look at me or run towards me to mark and toss. Then she had to run closer to me. Then run to me as in within a meter from me.
At this point I started giving her the chin to hand cue because I failed to shape it further. I marked and tossed even if she dropped the toy. I predicted that she would start predicting a hand touch and offer it still holding her toy. She did. I reinforced this and one day I could leave the other toy home.

 

I def think using two balls/toys maybe would be better. That's how we initially honestly taught our dog that fetch doesn't happen unless he drops the ball. Fetch is our dogs life....lol. But he def had the issue that you have where he'd run and get the ball, bring it back, but then not want to give it up.
I just used two balls. He drops the first ball like 4 ft from me if he sees I have another one all ready to go. But over time, I paired him dropping that ball with the drop it command (including working on drop it at home when he's got consumable treats and toys, etc.) and eventually he understood that fetch doesn't happen unless I drop the ball. And even still sometimes he doesn't drop it when we give that command. We just sit and take a break then - usually he's tired and that's his way of communicating I'm not ready to run after it again yet. He'll just keep the ball and chew on it.

**Just bc we had this issue with our dog - don't let your dog just chew on tennis balls like they are a chew toy if you aren't actively playing fetch. He broke one down and swallowed it once. Also it can be abrasive to their teeth. So we got a rubber ball that we use now - and also make sure it's not damaged enough that he could swallow it.

EDIT: also maybe your dog doesn't really like traditional fetch...and that's ok too.

 

I have had/trained a few dogs in my life, but this guy is stubborn on one topic.
He is a 7 year old Chihuahua, very happy positive dog with no aggression towards people. We have had him since weaning, and all other training has been succesful.
He has some weirdo traits, for example, he isnt responsive to treats. Generally he will take one, but wont usually eat them. He is far more responsive for praise.
Having said all that, I cant break him on fetch. I will pick up his favorite toy, toss it, he will chase it down and chew on it, pick it up and then drop it several feet away. Then my response is nothing, I stand my ground while he runs up to it, jumps on it, looking at me and just pushes it around with his nose. Stubborn kicks in and he will stare me down. This can go on for 15 minutes plus. When he finally gets it to me is when I will pick it up and throw. I never give in. This has been the normal for years now.

What are the thoughts on overcoming this? Its a real test of wills, but best I can come up with is that he wants me to chase it too.

You throw away the toy. You obviously don't want it.

You are not trainable..?