I don't see it as struggling with the tires at all. Its new to him. He went further than I thought he would on day 1. I'd call that success.David, watch that video! Near the beginning you jump into and then out of the box and so does he!
He clearly has no issues following you and I like seeing how when he struggles you back off and go back to where he is comfortable. I think that is important for confidence and trust. It's something a lot of people miss. There is a balance between letting them try and beating down the confidence.
Edit: Since the tires seem to stump him would you consider laying them in a single level on the ground for him to climb around on and in before stacking them?