That was bad advice. You don't need to remove leaves, just spray.

I don't really see wha tyou can do other than cross your fingers and hope it grows some new leaves to replace those lost. I had a different problem with a fruit tree that really did require me to remove affected leaves, and it was most of them. They did grow back.

But yours is a newly planted tree. Usually a bad infestation is a symptom of an underlying weakness with the tree. If your tree does die, being a first year tree, I would complain to the supplier, a reputablr one will give you a replacement, though may have to wait till the winter. Perhaps not mention the deliberate deleafing.