Hi- your tree is infested with wooly apple aphids. This is a significant pest that also feeds on roots and can weaken unprotected trees. https://entomology.ca.uky.edu/ef219

The under-sized, off-color leaves, and branch dieback also suggest a general failure to establish due possibly to poor site or plant conditions. Apple trees are vulnerable to a host of insect pest, mite, and disease problems. Based on the tree's current condition and the challenge of controlling wooly apple aphid you may want to consider removing the tree.

Alternately, you could spray the tree with a dormant oil spray now to control live aphids- don't worry about spray injury to the remaining leaves since they will be dropping soon. The waxy coating they produce makes insecticide control more difficult. 

https://www.purduelandscapereport.org/article/dormant-applications-horticultural-oil-can-help-control-troublesome-pests/

And then assess the tree in spring. It may leaf out in spring but lose its leaves quickly due to insufficient food reserves in the root system. If that occurs, it will not recover and should be removed.

Information on apples, including pruning:

APPLES

• Growing, Selecting, Root Stock, Spacing, Planting Apple Trees
• Tree Fruit Purchasing and Planting
• Watering, Fertilizing, Harvesting, 
• Spray Schedule Apples
• Training and Pruning Apple and Pear Trees
• Apple and Pear Diseases
• Apple and Pear Insect Pests
• Apple Problems not caused by Diseases or Insects

Jon