No one else on the Lagoon has experienced itching as the onset of change, so it's possible that Aiden wouldn't notice the darkening of his freckles through the intense waves of itchiness he experiences throughout the day, his claws darker than usual and, if pressed too hard against his skin, may cause himself to bleed.
The first of his changes hits him hard--quite literally. It feels like someone has punched him in the nose, flattening it out as it darkens and becomes moist, his whiskers bristling as they thicken and multiply. Something pulls on his ears, and they stretch out further, but the itching is brought to it's height when thick tufts of fur sprout from bare skin, forming sideburns down his face and halting just below the shoulderblades on his back.
Unfortunately, the fur doesn't stop there, and his arms seem to become thicker with the climbing fuzz, his two middle fingers fuzing somewhere along the way as the webbings thicken. His feet, too, begin to change, but no fur comes from them: instead, they thin and hollow, stretching until they are barely skin and bone, no longer suitable for walking on but rather useful for swimming.
The fur comes back, starting at the middle of his back, spreading downwards until it reaches his tailbone. At this point, his spine begins to stretch, and it painfully pulls forth to about a foot's worth of a fuzzy, fluked tail, all skin and bones at first, but as time goes on and the tail grows to full length, Aiden will notice weight gain in the form of his new tail's blubber.