First off, kudos to you for giving this dog a chance and willing to put some time and effort into him.
I think for now you need to keep the retrieving part of this dogs training as fun and positive as possible. The dog really needs FF but might have to try a different route and certainly not at this time. Keep building his confidence and trust in you.
It sounds like this may take some time and patience on your part.
I would try to get him going on bumpers first but If you just can't get him going I woukd see if birds light a fire in him.
Check this little video series out about building retrieving desire. It can be done with an older dog as well.https://m.youtube.com/watch?v=nj5y3z7THH4
I would then focus on teaching "hold". Make sure his sit is solid so he won't move around on you. Place bumper in his mouth and make him hold it and praise till he starts to understand. If he drops it say "no" and put it back in his mouth. It may take many sessions but he should eventually get it.
You also need to teach the "here" command. Put a rope on him so you can make him come every time you give the command.
If you can teach "hold" and "here" you'll be on your way..,
So with this dog it sounds like you need to keep things positive and fun but once he learns what is expected you can keep your standards high.
I would implement an e-collar but personally with this dog I would try and use a Hillmann approach of using it.
It's very light and positive.
Check out his stuff on YouTube. I think your dog would be a good candidate for this style of training.https://m.youtube.com/channel/UCEyAU4OEQsQAKJCildYrb9A
Now for the gun shyness issue.
If he gets excited about birds I would use this to your advantage. Get a wing clipped pigeon and toss it for him and have someone 50 yards or more away from you fire a .22 or something similar. Gradually move closer. Associate the gun with something fun is about to happen. Every time the gun come out he gets a bird.
If he won't get excited about birds then I'd be thinking he may just need to be pet. It would still be worth your time and effort IMO to obedience train him as well as retrieve.