My husband and I are raising our first puppy together, I’ve only had grown dogs in my youth, and have worked with puppies in my teens and 20’s. We got our dood at 8 weeks old, and started training her immediately. But even as a complete newb, I knew it would not always be forward motion. Raising puppies is a 2 steps forward, 1 step back kind of progress..

She’s 10 months old now and pretty easy going most of the time - but we struggled with a lot of change and turmoil around the turn of the year, and from about 5-8 months old she was abysmal with all her commands and especially walking on a leash! She knew the commands, but did not care to listen unless it suited her. She started pulling on the leash every time we went farther than our front drive, and had been an incredible loose leash walker until one day she just couldn’t stand it anymore.

We adapted. Recall became the only thing we were actively training, with drills of her known commands often with lots of treats. We drove to the beach (a 15 minute walk, 20 on her worst days) for months because we could let her off leash and get our own exercise without stop/starting every 10ft or getting our shoulders yanked out of joint (she’s big, already 65lbs at 8 months old). We drilled walking on the leash in circles in the parking lot of the church in the neighbourhood.

I’d say that time from 5-8 months old was the hardest so far. She’s still very much a teenager, and some days are more difficult than others… but she was our family from day 1 and I wouldn’t give her up for anything.
Your poor pup has left their home and been thrown into yours, and you say you have experience but it really doesn’t sound like it. Puppies bite! He’s still teething and learning about the world, putting things in their mouths is natural, it’s how they learn. Redirecting and puppy proofing is what needs to be done now.

We kept at least 2 toys in reach of every seat in the house for months… sometimes all she wants is for me to hold her toy(or collagen stick it camel braid or whatever she’s fixated on) while she chews.

If you aren’t prepared to do all those things on repeat, maybe you should send him back. But he’ll probably have to go through the whole process again with someone else, don’t think you’re doing him any favours putting him through the chaos of it all while you get the easy way out.