Certified GKH Gardening Expert
This is a problem. They should not be losing leaves until the fall.
This can be caused by a few things. The two most common are fungus and underwatering.
Treat the tree with a fungicide, to take care of any fungus. Make sure that your destroy the fallen leaves. Do not compost them - either throw them away or burn them.
Increase water to the tree as well.
It should try to regrow its leaves once the issue is resolved, but may not be able to before it is suppose to go dormant. It will need some extra TLC if this is the case.