The phrase "assured his position" is correct and usable in written English.
It can be used when discussing someone who has confirmed or guaranteed their role or status in a particular context, such as a job or organization.
Example: "After the successful project completion, the manager assured his position within the company."
Alternatives: "secured his role" or "confirmed his status".
Howard Dean raised at least $15 million in the fourth quarter, bringing his total for the year to more than $40 million and assuring his position as the leading fund-raiser in the Democratic presidential race.
But as soon as he was there he had many helpers to assure his position, for all the place-hunters, male and female, flocked to the doors of one who was so close to the ear of the most powerful.
Along with Ralph Manheim, Martin Esslin and Eric Bentley, Mr. Willett was one of the people primarily responsible for bringing Brecht to the attention of the English-speaking world and for assuring his position as a seminal theatrical figure of the 20th century.
Smith?s goal is to assure his own position as first author on a publication.
After 1918, this was no longer possible and every appointed Regent was assured of his position until he completed his term, resigned, or died.
Froome, however, felt assured by his position as first among equals at Sky, after offering decisive support to Bradley Wiggins in last year's Tour de France.
By 1299, Osman I felt assured of his position to declare himself Sultan and thereafter his territories became known as the Ottoman Empire.
Toward the last years of his reign Władysław IV sought to enhance his position and assure his son's succession by waging a war on the Ottoman Empire, for which he prepared, despite the lack of nobility support.
When his squad returns to training at the start of next month, though, Ferguson will be assuring Nani of his position in the Scot's plans for the future, and trying to convince his players they can meet the challenge Barcelona set with their devastating Wembley performance.
Even so, they felt compelled to quickly assure religious conservatives that his position had not changed, reaching out to leaders like Tony Perkins, president of the Family Research Council, an advocacy group that vigorously opposes abortion.
