If I understand this right - Grim is only 4 1/2 months old and now he has a martingale collar (of course it is too heavy for a pup) and that will be used to help him focus. Whatever happened to puppyhood? Isn't this supposed to be the fun time - let the puppy explore. Why not consider putting off the martingale collar until he is older and the weight won't bother him, and instead use a martingale harness (nylon and lightweight) that will help with the pulling issue and should work nicely with his new leash.
This is very hard to describe accurately. One- because I've never had a shepherd with this mindset and Two- when I try to explain things it somehow gets muddled and what I intend to say never seems to come out correctly.
I've had shepherds who were a little on the pushy side, but I've never in my life had one who seemed to have the teen 'butthead' phase coming out of the box. When on walks, this guy is at the end of the leash, head down the entire way sniffing. He's not on a walk, he's on a mission. The mission is to walk as fast as possible, all the while nearly choking himself on the lead. I don't want him to do damage to himself. What I'm left with is two choices. Either get it through his thick skull that he has GOT to listen to me (even though the way he wants things isn't the way I want things), or I stop walking him. I have at least 4 unsecured disks in my back. The entire lower part of my back is screws and rods. That part is OK, except for the muscles. I now feel like I have a turtle shell back after this last walk. I've never had a pup that NEVER put its ears back or looked like "Uh oh, I did something wrong". He is so full of himself that trying to bring him back to reality seems nearly impossible.
Now his breeder told me before I got him that he was 'stubborn' 'wanted things his own way' and that he was 'full of himself'. However, I thought surely a PUPPY couldn't be that bad, right? All of those really hard headed behaviors don't come up until they're older, right? WRONG. Now I can just not walk him until we get to the trainer's. Or, I can try to get through to this boy. I've had shepherds for over 20 years. I've never met one like him, though. I'm not new to them, I'm not new to the variations in temperaments. However, I can't for the life of me figure him out. If he WANTS to do something, he can be very biddable. If he doesn't feel that what I want him to do is worth more than what HE wants to do, though... you can forget it. I've seen and had confident pups. However, I've never had one before that thought he ruled the world. He will no doubt be an EXCELLENT working dog. However, the methods that will have to be implemented to get him there may rub some the wrong way. That's OK. No one else has to work with him, live with him, or get him there except me. I will be very interested to see what the trainer thinks of him. He's changing very quickly away from the 'puppy' he came to me as and into the 'adult mindset' that I thought I wouldn't see for at least another year.