It appears that the leaves have been skeletonized by an insect pest, probably a caterpillar (larva of a moth), or a beetle. I'm not sure which one without seeing a close up.
They will have a feeding stage of the life cycle, and then go into pupa stage and the feeding stops. So inspect the leaves top and bottom. If the critters are still there, you may want to spray for knock down in order to preserve what's left and allow re-foliation this season. If they are gone, then the tree is past danger of further feeding this season. But a resurgence could occur, especially next year at the same time. Without an identification of the pest, that's the best I can do. You could call a local tree service or pest control company for a diagnosis and treatment proposal. The tree is large enough that you would need a professional with spray equipment to reach the top.
