Let's say a child goes full-on Tarzan from 2 years old. How the heck do you show his intelligence? Or her. Whatever gender.
And not through time skips, by the way. It has to be slow.
Hmmm, interesting, let's see,
We need decorations!
Have the mc not just barely surviving, but show him as a connoisseur when it comes to arts. Sand castles, extracting pigments from flowers, stone tools, clay pots, wall paintings, planting flowers, keeping a note of everything.
If you are making your story in a diary format, it would be best to end every entry with a little philosophical thing, like the moral of the story kind of stuff.
I just got reminded of Pickle, a stone man from the Baki series who had quotes like that, paraphrasing him:
"I once ate a bee, and that set my mouth on fire. I felt like I was going to die. No amount of water could quell that heat. On that day I learnt that some things can kill you even when they are dead."
"The Nature's order is that big eat strong. But I hunt the big, I am an anomaly, hence I am alone in this world."