Seems like you're using a step-in harness. Which is great!
Can't tell too much with the pictures, is the harness loose? Does it move around easily? How many finger can you fit under the harness, both back and front. If it's loose it might be going to his throat area... have you watched where it goes when he's walking.
Also, he could be excited like the above poster said... hopefully someone will have a better idea.
You should also train now to not pull when you walk while he's young. They learn quick!
__________________ Rachel and  Mahina |