Your trainer is your best resource since he can actually see you and your dog together and recommend the best strategy.A couple of thoughts for you to mull over...
He's teething and his mouth is super uncomfortable,therefore the biting resurgence.Frozen cloths or large frozen carrots will give him some relief.If you aren't able to redirect him to an acceptable toy,stop playing and give him something to chew by himself instead.It'll be over soon!
The loose leash walking sounds like you hit the nail on the head....you gave up too soon.What you were doing with very short sessions successfully is exactly right.Don't give up!Duration comes with time and patience.A few seconds will become a few minutes and so on.
Excellent idea to reward with off leash running!Rewarding with what he loves best is the best motivation.He may need more opportunities to run and trot around at his own pace to expend that puppy energy.It will help him to settle easier in the house.If he's not able to relax and settle indoors currently,I would keep crating him for the time being if that's what helps him get in "chill mode".
Just a few ideas for you that hopefully help
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