you could maybe try a Halti it looks like a muzzle but isn't, it's a device to stop any dog pulling,
i recommend it for Boxers.

it fits over the head and the lead attaches quite high up, therefore giving you the control, if you have control of the head the dog will follow, lol.

i had a friend i bougth one for her Boxer and within days her children could walk him, she was amazed.

try any pet shop, they usually have them.
chell
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