Preffered transformation course: I would like his spine to extend, into his tail and his legs shrink into fins on his tail
As far as the art goes, here is what I would like to see. [0%} completely open, just arrived is fine.

25%
For the first transformation, he would be completely disconcerted with what was changing, and most likely be staring at it.
Webbed fingers
Very beginnings of his Horns/antennas
Cartilage reshaping in his ears
Small amounts of scale growth on his arms, patterns begin faintly
Small spines appear on his arms
Growth of barbed spines on his spinal column
Beginning of his tail (spinal growth)
Non-functioning gills on neck
Toes fuse with toenails and begin to web
50% His appetite begins to change, he becomes voracious. He begins to hunt fish, but he still retains some of his other diets, (When you grow up on Italian food, not even science can alter that.)
Unsure if he can believe this. Continuation of changes He is losing feeling in his legs.
Continuation of growth of his horns/antennas
Cartilage reshaping of his ears, opening of his audio receptors
Scale growth covers his body
Continuation of growth of his spines on his arms, and spinal column
Tail grows out to about 50% of its eventual length and width
Gills are semi functional
Feet turn to flippers

75% He can no longer walk like a person, he begins to slither.
Continuation of growth of his horns/antennas, by now he is accustomed to it.
Ears finish reshaping
His patterns darken, and his skins completely gone, replaced entirely by scales.
Spines on his spine and continue to grow
Tail grows out to about 75% of its eventual length and width,
His legs fuse with his tail, and he loses all mobility with them.
Gills are completely function; he switches to breathing through them
Flippers become fins
Facial bones bulge outward
100%: The changes Finish and he is finally come to terms with his fate, he doesn’t fight it at all.
Horns/antennas are done growing
Patterns and scale coloration finishes
His spines finish growth, ending with five. The spine on his elbow is Twelve inches, and each one is smaller by an inch then last one as the go from elbow to wrist.
His back spines range from 2 feet to 5 inches as the go up and down his back.
His fins on his tail are also poisoned and webbed.
His tail has completely finished, and has enveloped his legs completely.
Gills are his only way to breathe
Face finishes, about 50% lionfish and 50% human