May be he's just not a dogs dog
We did everything by the book with Bella and still seemed to fail her, she was always timid. Puppy classes were a no no. Bella met with other puppies and basically as they were usual boisterous puppies made her worse.
We went to obedience type training, although we didn't achieve much as she could already sit, stay, recall etc we kept on for approx 10 months, dogs were all on leads and it just got her used to being around dogs in a controlled environment.
Until we got Molly Bella never socialised with other dogs, that was her choice, on walks she'd walk around them, if I was talking to other owners she'd sit patiently nearby, on meets she'd just fetch her ball and ignore everyone else
Whilst out how many dogs do you see actually socialising, not actually that many, some dogs just plod along with there owners, others are very ball orientated. Not many round here actually play.
Is yours a worker, it might be he would rather fetch things and be trained with balls etc than socialising with other dogs.
I have Molly too who is the complete opposite and she's one of those dogs I used to dislike those who 'just wanted to play'. But even now Bella doesn't play like many dogs fetch is her favourite and she'll chase Molly around when she gets the toy first but that's it, at home she very rarely plays.
I would carry on with some training, find out what makes your dog tick and enhance that, if you know any older dogs that he could socialise with. Going on col meets, they are great for socialising and owners are respectful of other dogs
I could go on but have tea to cook
Where do you live?