Can anyone please help me identify the main theme in this poem and its significance?

Slow Reader by Vicki Feaver

He can make sculptures
And fabulous machines
Invent games, tell jokes
Give solumn, adult advice
But he is slow to read.
When I take him on my knee
With his Ladybird book
He gazes into the air
Sighing and shaking his head
Like an old man
Who knows the mountains
Are impassable.

He toys with words
Letting them grow cold
As gristly meat
Until I relent
And let him wriggle free –
A fish returning
To its element
Or a white-eyed colt
Shying from the bit
As if he sees
That if he takes it
In his mouth
He’ll never run
Quite free again.

thank you

The theme is expressed in the last four lines. To what does "it" refer?