The lower arm is too thin compared to the bizeps.
Looks like the guy only trained on cable machines.

The with of the lower arm should be as wide as the hand.
(At least on my arm it looks like this)

you should rework the thumb also.
He would have a hard time holding something.
the thumb should be turned 90 degrees to the hand.
And it might need to be a bit longer (for an average hand)