The plant is in transition from water roots to soil roots and may not have sufficient soil roots to support the number of leaves. It may benefit from a humidity tent or humidity tray to minimize moisture loss via transpiration. (on second though, stick with humidity tray -- the location is too sunny for a humidy tent). At the very least heavily mist until dripping with filtered/unchlorinated water every morning.
Avocado needs well draining soil. I usualy add about 1/4 to 1/3 sand. Avocado is sensitive to overwatering, especially during winter. You may want to wait test by pushing your finger into the soil to first knuckle. If testing just the surface, wait until all hint of moisture is gone -- I.e. soil does not stick to your finger. This depends on the kind of container and number/size of drain holes as well.
If the plant is on the windowsill, are the leaves touching the glass? It can get too cold that way, Especialy if you are pulling window covering over the plant and trapping the cold air.
Oh, one more -- has Mr. Avocado always lived in this window? If not, the sudden transition from somewhat shadier location may have been too much, especially since winter sun is pretty direct into the window. Avocado sunburns easily and needs to be gradually acclimated (especially in spring when you take it outside for summer-long vacation
Thank you guys for your help! "Mr. Avocado" (yes, only grown for the enjoyment of growing him!) is in a black, plastic 8 inch deep, with a 7 inch diameter, pot, that has 16 drain holes (all on the under side) that are big enough to fit my pointer finger.
Yes, he is in the same window seal he was started in, and at night, when the sun goes down, we move him to the middle of the table, where the room stays 65* F at night. (we've done this since he was rooting in water.)
He does not touch the glass of the window. It is winter though, perhaps I'm over watering him and not giving the lower part of the pot enough time to drain/dry, as to not make him sick!!!
I last watered him 2 days ago, I will wait until the first knuckle deep dirt is dryer, and not moist.
Oh, and the 2 weeks before he was transfered to dirt, we added his soil to his glass jar he was in, one tsp at a time until there was almost 2 inches of dirt at the bottom that his roots were touching.
And his roots were big, thick, and they were just about 5 inches long..... If this helps any more.