I am not sure where you live or what your weather has been. That being said...young avo trees are very sensitive to hot/dry weather. They sunburn easily. After 40 years of growing...I still had a young "Reed" get cooked last week during our recent SoCal heatwave. Your arborist is probably correct. If so, provide shade during heatwaves and keep the soil moist.

Thank you for responding. I live in Southern California, Torrance area, and the past week or two hasn't been very warm. It is in the corner of the yard by a wall so it doesn't get a crazy amount of sunlight. Can it be sunburn if it happens before the leaf even grows to be 1/2 inch? It just looks so sad with zero leaves on it. Will it get better and if so how long do you think before a leaf grows full size and healthy?

Yes, younger leaves are actually the most sensitive. I grew up in Westchester...and I know your weather is good for avos. But, this Santa Ana condition (coming in again tomorrow) is rough on the trees. Actually, dwarf trees are the most vulnerable because of their limited roots to take up moisture. Your soil is probably pretty sandy...so you need to be diligent about keeping your soil moist. I have shadecloth that I use on occasion to combat intense sunlight during heatwaves. Also, avos do shed their leaves (all at once) in the Fall (usually). ED of Somis