As far as healthiness go: that looks like a very healthy young tree, good job taking care of it!
Maybe someone else would give you different advice, but if it were my tree,
I would prune all those leggy branches hard, about 2/3 of them. I know it's sound very drastic, but cherimoya is a very vigorous grower in Southern California, it'll make your tree bushier and rounder (thus: more fruits); so the branches will be stronger, and as was already mentioned, easier to pollinate the flowers in the future.
My trees are behind also, compared to Simon's; the largest fruits are only about the size of a mandarin/tangerine. Here are some of my annona photos:
Dr. White (Thanks to Simon for the scion!):
El bumpo (Wondering what caused the fruit to "deformed" like that?):
Orton (Prolific producer):
African pride atemoya (On this particular tree, I had very low percentage of pollination taking in the first few weeks of flowering this season; as well as the previous 2 years):