If you can find some garden- phos mix up 2 tablespoons to a gallon of water and use as a soil drench , you'll have to do this several times . I would do it now and in a couple months do another treatment , and continue this until you see new shoots emerging. Avocados are one of the most sensitive plants that need good air exchange around the roots to be healthy. Also mulch it heavily. The feeder roots like to grow up into the mulch and feed on it . Leaves , wood chips are really good to use for mulch. If the tree does not make it and you want to try again I suggest planting on a mound using lots of sand mixed in to create a good draining soil.
William