I think it is great that all he does is walk around in harness.
You need to be carefull that he does not pull any weight at his young age.
What a good job for him as he matures to help in the garden, it will teach him patience too.
As long as the harness fits and you do not put much weight behind him, 25 lbs or so max, at his age and size, he is still growing, I would keep doing it.
My friend taught her horse patience, by harnessing him up and ground driving around her big stable to water horses, he had to stop and stand at each bucket while it was filled.
I would love to see a photo. If you don't want a critique you better pm me.