I had a Border Collie cross Belgian Shepherd a very long time (we’re talking decades) ago, that was just the same as a youngster.
Village hall training was absolutely the wrong environment for her, she took forever to mature and at 10 months we had the lovely double whammy of teenager plus high energy dog that still hadn’t nailed the basics.
To this day she is still the only pup that has reduced me to tears. The world was far too exciting for her and she could not focus on me or listen. (Note I said could not, not would not.) I had to train her in low stimulus environments only for the first 12 or so months and manage her behaviour elsewhere, as she wasn’t sound enough on the basics that we needed for control anywhere with other dogs, people or wildlife.
What worked was calm handling, consistency and wearing out her brain instead of her body. Crucially it had to be fun and motivational, but not too stimulating or exciting. It’s a tricky balance, but it can be done. Honestly though, remaining calm and being consistent were the biggest factors, that and her finally maturing. Training basically had to be low-key, with high value rewards - her absolute favourite was eye-wateringly expensive turkey sausage. We also had one particular squeaky toy that lived in my coat pocket and only came out on walks, that was her absolute favourite.
If it helps, she turned out to be one of the best dogs I have ever had. As an adult she would walk next to me off the lead in parks full of dogs and people and ignore everybody and everything. She coped with my 3 dc coming along one by one with grace and was the sweetest, most gentle, affectionate girl with them.
If he isn’t ready for you to reduce the treat rewards, then don’t. Just make sure, for when you are out and about that treats are the absolute best thing ever and broken down into tiny pieces. I don’t agree with trainers who say you shouldn’t treat to reward the behaviour you want. Why would the dog want to essentially go against their instinct and nature unless it’s really rewarding for them? Watch some Kikopup or Chirag Patel videos on YouTube and notice how often they treat.
Also, pick your battles. Do you really need him to be obedience trained or do you just want a calm, confident companion, with a strong bond to you who knows he needs to come back when he’s called and will stay with you when required, rather than one that can produce the perfect square sit or lie down instantly when you tell him to? A lot of dog training classes are obsessed with sit/down/micro-control and it leads to people thinking their dog is only in control if they have made them go into a certain position. (I also think a lot of badly run ‘local’ dog training classes can damage the dog/person bond by some of the methods they use.) I think/hope the days of dog ‘obedience’ are numbered as people start to realise dogs needed to be treated with more respect and allowed choices, rather than effectively subjugating them into behaving like robots. Have a think about how you would like to live with him and focus on the things you need for your life and relationship with him, rather than what a local dog training class insists are ‘essential’ behaviours. Relationship based training is the way forward. (If you want to learn more about this look into Suzanne Clothier and look at Chirag Patel’s training.). Ultimately That’s what I did with my girl, trained what we needed for our life, rather than what other people thought she should know/do. I had other dogs at the time that had been trained using more traditional, although still positive methods and passed all sorts of ‘awards’ and certificates, but that wasn’t right for her and I needed to respect that.
If you are feeling completely hopeless (I do understand, been there done that) about training, maybe a few sessions with a positive trainer, rather than soldiering on alone? They will be able to observe, advise and help you train in the real world at the level your dog is able to cope with, rather than in close proximity to other dogs in a small village/church hall. The Institute of Modern Dog Trainers is a good place to look, as they are committed to only using science based, force free, motivational training methods and required to do continuous professional development to ensure they are up to date.