The first time Stoick admitted to himself that his son was different was on Hiccup's 5th birthday. The boy was old enough to start learning the ways of Vikings, and Stoick was eager to gift his son with a wooden sword. He thought the boy would be excited to take up the weapon and play fight like the other kids of the village, but Hiccup looked at it then set it aside to go back to his tinkering.
Stoick had tried to brush it off. The boy was still young, and he'd come early. Maybe he wasn't as advanced as his peers. Maybe his interest in the Viking ways hadn't caught up with him yet.
The second time Stoick admitted to himself that his son was different was when he took Hiccup fishing at age 6. He'd planned to bond with his boy over how to bait a hook and cast and prepare a catch. But Hiccup had sighed loudly and lazed around on the grass before finally running off to hunt for trolls. Trolls, for Thor's sake! Something that didn't even exist!
The third time Stoick admitted to himself that his son was different was when Hiccup built his first contraption at age 7. The boy had proudly presented it to him one evening. He'd chattered nonstop about how the model prototype that would lure dragons in and snap a net over any that triggered it. The boy meant well, Stoick knew, but building such elaborate contraptions would take up too much time and resources, when picking up a weapon and hurling it at full force at a dragon was simple and just as effective.
Stoick finally gave in to the fact that his son was different. The boy's hands were always going, always fidgeting. Instead of wasting time creating all those useless contraptions, Stoick knew just what to do to keep his son's hands busy.
Hiccup was 8 years old when Stoick walked him to the blacksmith shop and presented Gobber with his new apprentice. At least there, the boy could put his busy hands to good use. Maybe making weapons would divert his interest to more Viking-like things. Then maybe Stoick would finally have something in common with his boy.