If it is on a slope then the drainage should be pretty good. What is your soil type? Is it clay? Did you amend the soil when you planted? (Usually not recommended for trees).

How hot has it been? It may simply be responding to hot weather. It is a young tree and vulnerable. A first year avocado tree tends to look pretty bad until the root system starts to establish. You might need to rig some temporary shade for the young tree until autumn. The root systems are sensitive, thready, brittle, and like to be shaded. A mulch will help cool the roots (keep the mulch a few inches away from the trunk). As the tree grows and matures, they like to have a thick, fluffy mulch of their own fallen leaves.

It is clay. I did amend the soil with compost in the planting hole.

I think, despite being on a slope, it likely has root rot... I've dug a little down under the tree today and the roots look brown instead of white... :-*(

Yeah, too much water. Am in SoCal with a couple on a hillside with afternoon shade, and am watering once every five days. In the summer, I'll do every 3 days.

Protect it from the hot sun by installing shade cloth canopy over it.