I'm not that familiar with dobes, and what they were initially bred for other than protection! It is typical puppy behavior but for some breeds it's more prevalent, like my GSD!! Who is 5 months and everything goes in the mouth! She doesn't really bite down either, but has to have it in her mouth. Luckily she doesn't chew at this point, she just carries things around and puts them where she wants them (that's the herding coming out). But because of the training that I'm (hopefully) going to try with her I can't make it a negative thing, so I just try to redirect her as much as I can! Good luck, I'm feeling your pain....litera lly!!
Oh, when you pick him up and he struggles, DON'T put him down until he's still for a bit. To a dog that's a submissive position and when they struggle and you let them down immediately they've won, and in some way in their mind they are not submissive to you! I learned that too late with my first GSD, and by the time a trainer told me that he was already too big to hold and a total butt!! So I ended up having to correct my mistakes, not his fault, all mine!! Again good luck!