When you break off a twig is the wood still green? If so then it may still have a chance.

It may have been stressed by improper watering or another issue for a newly planted tree. I've had plants (rescues from the town dump left by a wholesale nursery) that had the entire top dead but that resprouted later from the roots. So scratching a branch will tell you if the branch is alive (green inner bark,) but unless you are in a real hurry to replace this plant, I'd just leave it until June of next year and see how it does. If it never leafs out, remove it and in September of next year, replace it. Many plants do better with fall planting since the soil is still warm, but the top part of the plant isn't stressed by extreme heat or dry weather. Plants have the fall to grow more roots before they need to support next year's leaves.

I'd go with the twig test. You don't need to break them off, just scratch the bark of a twig. If its green, then its alive.

I doubt you want to wait for the whole tree to regrow from the roots.

PS - If the first one is dead, test more until you either find one that's alive, or have tested enough to be relatively sure that all the twigs are dead. Then scrape the trunk bark off to test that.

Leaves falling off in Spring isn't a good sign, I vote that it's dead. The bark test is good, also look for fruiting bodies of fungus like conchs.