Use some frozen birds for a while on retrieve training. Just encourage him to bring it back promptly, followed with praise.
I am NOT an authority, just an old guy.
With my dogs, I do kitchen training: close off the kitchen with me and the dog. I sit on the floor, holding the dog's leash (well, sitting on it.)
Take a frozen feathered quail/pheasant, and work on "fetch." Dog chases, grabs, starts mouthing, I say "no." Dog ignores me.
I repeat. Dog thinks I'm a tease, but unstable and perhaps it should obey. Every grab and and chew..."NO." Dog is now wondering how it can get to a phone and call for psychiatric help for me. If dog ignores me, quick leash tug reminds dog of our connection.
We continue until I get two in a row.
We stop, dog gets treat. Bird back in the freezer back, kitchen taken off lock down. Dog out on the yard line, after extravagant praise.
Couple times a day, minimum; takes about a week to be steady.
Sweep the kitchen floor for evidence of frozen bird work.
(If dog tries to get a couple of chews in before I get there, then I stop throwing the bird and just open dog's mouth, order fetch, and insert bird. Command "out" and remove bird, praise dog, repeat three or four times. Then back to toss and "fetch."
My premise is I don't have to be smarter than dog, just more stubborn. As the dog steadies down to the role of hunter/retriever, it is convinced I'm "not quite right" and all the dogs I've owned and trained for friends, have brought back live birds to their owner upon occasion.
However you do it, have fun with it.
Best wishes.