Goldens are a nightmare for mouthing. I've had dogs my entire life (Flat Coats, GSDs, Spaniels) and the Golden was the biggest mouther I have ever met in my entire life. And mine looked like a vicious little bastard when he did it - used to get a 'devil face' on and genuinely looked quite aggressive. But it is just how they play with their littermates and how they think you want to play...
I would warn you it does not necessarily get better until the puppies are 6-7 months old (with any breed) because obviously you can't just exercise them into exhaustion and they have teeny-tiny baby brains for ages (bigger dogs mature more slowly).
That being said, I think of all the dogs I've owned, the Golden is the most loyal companion. It takes time, but I don't think I would get another breed now tbh.
In terms of tiring him out. Start training him on all the key commands; sit, down, stay etc. Get him used to being held and groomed, bathed and handled in general (balls held, ears inspected etc) - really important with big dogs they're used it to because trying to groom or have a vet check over a big dog that is nervous is a nightmare. If you have access to a garden, you can start doing recall and teaching him to walk to heel. Get him used to the lead.
Even though you can't put him down, you can take him outside. Take him for car journeys, and let him sit on your lap and watch people go by etc. You can hold him in your arms (carefully) or straight him to your chest and sit outside a coffee shop etc. Again, really easy and totally exhausts them.