Hi there,
heres my 2 cents
I did some intensive training with lots of yummy treats in my left hand. The dog is on the left, the line in the right hand and low down on my thingh was treats that I could give out slowly.
Another idea which has been mentioned is to vary your training with relax time. Train a bit, then find a place where you can stand and the dog, on a long line, can sniff around a bit. Sniffing around is very important for a dog, and it must be frustrating to leave the house and always be in heel position. When you´ve had enough, get the dog in heel and proceed with the training. Have a bounce in your step, a sing in your voice, some treats, and eonough time to stop and start, change direction etc. Make it fun, and then give a command that your dog may have a loose sniff around. I´m sure he will see the compromise with this situation
Good luck
Annie