There appears to be lot of dieback/dead wood on the tree. Did you have any strange spring weather? Like a hard freeze right before budding/leaf out? Did you have any droughty periods last year? What kind of watering practices do you follow?

It also appears to be in quite a lot of shade, which could also affect how densely it leafs out.

Other than the obvious thoughts (which I already mentioned above) I don't see anything that jumps out. The new growth looks healthy enough. Remove the dead wood and water deeply when necessary. And maybe trim up the surrounding vegetation to provide a bit more direct light.

Thank you SO much for responding!

We live in Maryland and Maryland weather never is normal. One day could be winter and the next summer it's really bizarre, BUT now that you mention it there was a period where spring came (march?), flowers started blooming, then BAM back down to the 30s! It was so drastic I remember thinking wow, this is even crazy for Maryland weather! I can definitely ask around to see if anyone else here is having issues.

How do I tell which branches are dead? Do I cut them off at the trunk?

I've never had a watering practice since nature has taken good care of him for so long. I'm guessing I should start one? I can't think of any droughts.

Thanks again!

Anything that is pale gray in color and has no leaves on it is dead. I can see quite a bit in your second and fourth photos. Sometimes it is not an entire branch but just a portion. Cut these back to a joint where there is better colored, live wood. Otherwise you can cut off any completely dead branches at the trunk (just to the branch side of the trunk to a swelling known as the branch 'collar').

I'll give it a try! Thank you sooo incredibly much!