Hi, ChocolateBiscuitCake, hope I can help.
Let's break this down into two avenues - firstly, helping your dog now that he is back home. Have a look at the APDT website, and you'll see lots of lovely trainers, who all use positive, gentle methods. Pulling on the lead is one of the commonest problems with a boisterous young dog, but it's also happily one of the ones that can really turn round quite quickly. I'm attending training class with one of our dogs at the moment and there is the most wonderful, nutty, young choc lab there. When we started training, only six weeks ago, he was a complete loon, and trashed the church hall on the first night. Within three weeks there was an enormous improvement and by week 6 - magic. That's from a good trainer, committed owners (who are completely besotted by him) who practice what trainer says, and going to class for an hour a week. Our trainer is fabulous - kind, positive, funny and we've all learned so much. I can honestly say that going to class is one of the highlights of my week. Lots of trainers (ours included) often also run simple one-off workshops that just focus on loose lead walking, since it is such a fundamental for dog ownership.
You are taking care of the veterinary issues, which is great. Make sure your vet is aware of the circs and is documenting/photographing him. Now that he's home, be gentle, don't push him, and just let him settle back in - he's probably stressed and confused. It's still too early to say if he has any long lasting issues as a result of what he has been through - just keep an eye out for any signs of stress or trauma. I'd also endorse what previous posters have said about making absolutely sure your DC understand the dog has and NEEDS clear boundaries - no disturbing while he's asleep or eating for example.
Now there's the issue of the kennel/trainer, and how that is resolved. There are several red flags from your initial post - for instance the fact that they haven't spoken to you for four weeks. A friend who is a farmer has sent her BC pup away to learn how to be a sheepdog and she and the trainer are conversing every three or four days, often more often. You should have been getting very regular updates, including things like emails, video footage, and digital pictures. Any health issues such as you've listed should have been picked up and discussed at length with you, including any ongoing treatment etc.
Some questions about your trainer. How did you find him? Is he a member of any professional training body? What happens if you google him and the kennel separately? Does that throw up any other bad experiences by previous customers?
There is no point at all in contacting the RSPCA. Generally, where a premises is licensed, they have an agreement with Local Authorities that they will not intervene/investigate, since their argument is that this should be picked up by local Council. Your local Council is responsible for licensing this kennel and therefore on Monday this should be your first port of call. Usually the licensing of boarding kennels is undertaken by either Environmental Health or Trading Standards. You will need to report this to the Council where the kennel are,not your own if that is different. Most Councils now have on their websites, downloadable info about the licences. When you speak to them, state your complaint clearly and concisely. Ask for the following (you have the right to all these under FOI) - how long has the kennel been licensed as a boarding kennel, copies of all previous inspection reports for the last three years, copies of any correspondence relating to the licence conditions. Ask if anyone else has reported any problems/made complaints? Remember and bear in mind that the Council cannot act on any complaint about the quality of the training, only about the health of the dog. If you need more help with this, come back to me.
Depending on the trainer's professional body, I would also consider making a complaint through them. Some "training" bodies are little more than trade associations, so complaints will have little impact, while others are more serious and professional. Which one is he a member of?
I also think that the cloak of anonymity is not helping current or potential future customers. I can immediately take a guess who it might be - does the kennel name start with R?
You might also find it helpful to speak with Beverley Cuddy of Dogs Today magazine. Beverley is a great campaigner once she is involved in an issue and this is something with a great deal of relevance for many dog owners. I am so sorry to hear you've had this experience, and am wishing you and your dog all the best.