Looking on the internet deeply has found these results:

He is not at his seat is the most popular phrase on the web.

He is not on his seat 13 results on the web	He is not at his seat 16 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
May 27, 2010 ... ... wrong because the pronoun �his� is missing. in essence, the correct phrases should be �he is on THE phone� and �he is not on HIS seat�. 7 Dis 2015 ... KUALA LUMPUR: Veteran newsman and Najib Razak critic, Kadir Jasin described the Prime Minister as �scared� whilst grassroots support�... He is not on his seat right now ???? ?_? ???????? https://twitter.com/ warofoneman/status/676443799028039680 � Idinagdag ni Ryoko,.	Parliament took note and, under Rule 4(1), established the vacancy of his seat with effect from 24.07.2008. I had almost overlooked one of them just now, but there he is striding quickly to his seat. Hannigan was in his seat all night. And gave up his seat for personal reasons. By which time he had been observed to change his seat no less than 5 times, always choosing a seat next to a female person. To not let anyone sit in his seat that day. He will take up his seat in Alexandria as supreme governor of Egypt and the eastern provinces. We're taking him back to his seat.

He is not on his seat

13 results on the web

He is not at his seat

16 results on the web

More popular!

May 27, 2010 ... ... wrong because the pronoun �his� is missing. in essence, the correct phrases should be �he is on THE phone� and �he is not on HIS seat�.
7 Dis 2015 ... KUALA LUMPUR: Veteran newsman and Najib Razak critic, Kadir Jasin described the Prime Minister as �scared� whilst grassroots support�...
He is not on his seat right now ???? ?_? ???????? https://twitter.com/ warofoneman/status/676443799028039680 � Idinagdag ni Ryoko,.

Parliament took note and, under Rule 4(1), established the vacancy of his seat with effect from 24.07.2008.
I had almost overlooked one of them just now, but there he is striding quickly to his seat.
Hannigan was in his seat all night.
And gave up his seat for personal reasons.
By which time he had been observed to change his seat no less than 5 times, always choosing a seat next to a female person.
To not let anyone sit in his seat that day.
He will take up his seat in Alexandria as supreme governor of Egypt and the eastern provinces.
We're taking him back to his seat.