I could write a book about my Bobby. I’ve had dogs all my life, none as complicated as a cocker. For a first dog , it means you have to follow a different rule book with him.
As your trainers have doubtless said, do not challenge or punish his guarding . Praise/treat him if he stops.
Before he has bitten, has he given a warning growl? If he has, good, respect him and back off. Ignore him completely . Even leave the room. He will be bored and want you, not the object.
Staring at a reactive dog in the eyes will cause more trouble.
Does he object to having hands on his head or collar for going out? I used a slip lead to literally drop over his head. It’s a rope noose with a knot or tab to stop it getting tight. You can always keeps lightweight lead on him in the house to grab if needed.
We had a problem with car…..he had a ramp to his crate in the boot but not let me lift him . If other people were coming too, he had to be kept out till last because he ‘owned’ the car .
Nonetheless I loved him to bits and 90% of the time he was perfect ! He even acted as a medical sniffer dog for my other dog with an auto immune issue, so I’d take her to the vet in time.
My behaviourist said that he was very shy and nervous and the adrenaline welled up in him and it can take even a day to subside and calm down.
My vet wouldn’t neuter him because he would maybe become more fearful.
One groomer was amazing with him, she could soothe any difficult dog to sleep. So that is another direction to follow. For now keep him happy and treat orientated.
We have a COL member/ behaviourist who specialises in cockers. Top Barks , in York .
I know how difficult this is for your family.