Sid felt himself beginning to shrink.
He was undergoing a shrinkage.
Sid immediately felt himself starting to become smaller than his usual height.
His clothes began to bunch in places, and he saw his goggles becoming looser, slinking down his face, so he took them off. He rubbed his brown eyes to see what he was seeing was true - he was shrinking.
He put his cap on [front first this time] and saw to his surprise that his cap seemed to be too big for him. He pulled it down, and it seemed to cover his eyes.
He saw his feet were hardly touching the ground, and his boots were too big for him. "Wow." Sid said, gobsmacked. "I'm shrinking." This was proved upon seeing his hands disappearing into his sleeves.
As if to prove it, he got himself off the sofa, and he saw his jeans looked much too baggy to wear - they immediately fell down. AND his black boxers followed suit.
He pulled off his baggy-looking socks, and stared at his bare feet, as he felt himself shrink before his eyes. He wriggled his toes. Luckily his shirts were like a dress to him now and covered his naked thighs.
He noted that he was no higher than the arm of the sofa he was sitting on.