The phrase "able to assert his" is correct and usable in written English.
It can be used when discussing someone's capability to express or affirm something confidently or authoritatively.
Example: "He is able to assert his opinions during meetings without hesitation."
Alternatives: "capable of stating his" or "able to declare his".
Arriving shortly after Garnier had been killed in battle, Philastre was able to assert his authority.
She died in childbirth and without heir; her husband was then able to assert his authority in Poland.
The victory also served to consolidate Charles's leadership of the Franks, and he was able to assert his authority in Aquitaine, where Eudes swore allegiance to him.
Because Aethelflaed had extended her influence into Wales and Northumbria, Edward was able to assert his authority over these regions as well.
Asked if he felt able to assert his independence from the governor's office as some past chairmen have, Mr. Sander chose his words with evident care.
Monthly resignations cannot be sustained if his office is to be taken seriously - and if it isn't it won't be able to assert his power over the capital when an economic recovery plan would be no bad thing.
Nothing could better please the American president and his advisors than to be able to assert that, in the wake of his Cairo speech, even Mr. Netanyahu had shifted ground and was open to compromise.
Accepting his situation, he is able to assert himself through a polymorphic sexual abandon in which the lines between the sexes dissolve, to the delight of all.
In the Austrian west, Duke Frederick IV of the Tirolean branch lost the Aargau to the Swiss but was able to assert himself in Tirol against a rebellion of his nobles.
They are looking for any clues about whether Kim Jong-un, the second son of Kim Jong-il's third wife, will be able to assert control over the monolithic dictatorship established by his father and grandfather.
Indeed, they may facilitate getting his testimony since he will not be able to assert 5th Amendment rights.
