The phrase "assure his position" is correct and usable in written English.
It can be used when discussing the act of confirming or guaranteeing someone's role or status in a particular context, such as a job or organization.
Example: "The manager took steps to assure his position within the company during the restructuring process."
Alternatives: "secure his role" or "confirm 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.
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.
This has already assured his position to a large extent with Sonia Gandhi and the congress managers being able to strengthen that support to evolve a consensus.
Smith?s goal is to assure his own position as first author on a publication.
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.
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.
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.
But he seemed to back away from that categorical position when he tried to assure the justices that his position would not convert into a taking every temporary delay that a property owner must endure while seeking a building permit under ordinary zoning laws.
