He's incompetent to do the job.
No one else can fill that job.
He is very enthusiastic, but his hand does not go together with his enthusiasm.
This task is too much for him.
Young as he is, he is not equal to the task.
his competent but dull performance
He is not equal to the task.
He is far from suitable for that job.
I don't think he is fit for the job.
He is incompetent at his job.
There is no future in his job.
He's inadequate for the job.
To do him justice, he is not equal to the job.
He's woefully inadequate for the job.