Gentle Leader Head collar or No pull harness

Hi! We are currently using the gentle leader head collar on our dog, Hachi (7 months old). We have been using it for about two months already but just recently he is starting to fight us on walks with it on. It feels like we are torturing him on every walk. We have also had it fitted on Hachi by our trainer so I know it’s not too tight and it is the right size. We have tried a lot of positive association with the head collar by giving him treats when we put it on but nothing seems to help. Our trainer is suggesting we try a no pull harness this week during class to see if we have better results. We have done some leash work without it but he is still pulling a lot without it so I was wondering what others have used with their dogs during training. Any feedback would be much appreciated.
Thanks!
Nicki