Donald Trump Jr has a previously undisclosed business relationship with a longtime hunting buddy who helped raise millions of dollars for his father’s 2016 campaign and has had special access to top US officials since the election, records show.

The president’s eldest son and Texas hedge fund manager Gentry Beach have been involved in business deals since the mid-2000s and recently formed a company despite claims by both men that they were just friends, according to previously unreported court records and other documents.

Beach last year met top National Security Council officials to push a plan that would curb US sanctions in Venezuela and open up business for US companies in the oil-rich nation.

Ethics experts questioned whether Beach’s access to officials and advocacy for policy changes were made possible by the president’s son’s influence and could also benefit the Trump family.

Noah Bookbinder, who leads Citizens for Responsibility and Ethics in Washington, said: “This feeds into the same concerns that we’ve had all along: the really fuzzy line between the presidency and the Trumps’ companies. Donald Trump Jr sort of straddles that line all the time.”

Last February, Beach and an Iraqi-American businessman met top NSC officials to present their plan for Venezuela , according to a former US official with direct knowledge of the proposal. Career foreign policy experts were instructed to take the meetings, first reported last April by the website Mic.com, at the direction of the West Wing because Beach and the businessman were friends of Trump Jr, the official said.

The official, who spoke on condition of anonymity, said NSC lawyers raised red flags about the meeting. The US did not act on the pitch, which would have gone against the president’s hardline stance on Venezuela and its president, Nicolás Maduro.

Seven months later, Beach attended a private lunch in Dallas between interior secretary Ryan Zinke and Republican donors including businessmen with petroleum interests, according to a copy of Zinke’s schedule. The interior department did not respond to a request for comment. A White House official said Trump Jr did not arrange Beach’s visit to the NSC and his proposal was dismissed.

In a statement, the Trump Organization said Trump Jr had never played a role arranging meetings “with anyone at the White House or any other government agency”.

Donald Trump Jr on Monday visiting a factory in Canonsburg, Pennsylvania. Photograph: Reuters

Alan Garten, the Trump Organization’s general counsel, acknowledged that Trump Jr had invested with Beach but referred to a statement released by the company in April 2017, which said their relationship was “strictly personal”.

In a statement provided by a friend, Beach said it was “absolutely not true” that he had ever “used my longtime personal friendship with Donald Trump Jr” to influence government decision making.

According to his friends, Beach, who has known Trump Jr since they attended the University of Pennsylvania in the late 1990s, developed his own relationships and does not need Trump Jr to broker introductions.

Beach was an avid fundraiser and campaigner for Donald Trump, particularly in Texas, where Trump Jr told donors last March Beach and another hunting friend, Tom Hicks Jr, raised millions, according to the Dallas Morning News.

Beach was a finance vice-chairman for the Trump inaugural committee and faced scrutiny after a nonprofit he started at the time advertised hunting and fishing trips with Trump Jr and his brother, Eric, to million-dollar donors.

Last October, Beach incorporated a business called Future Venture LLC in Delaware without listing any Trump connection. But a disclosure report filed with New York City officials and obtained by AP via a public records request shows Trump Jr is named as president, secretary and treasurer. The purpose of the company could not be determined. The Trump Organization said it was to pursue technology investments.

Previously unreported court documents show that the two men, each a godfather to one of the other’s sons, did business well before they formed Future Venture. In a 2010 deposition, Trump Jr testified that he invested $200,000 in a dry Texas oil well managed by Beach’s father and an undisclosed amount in a failed mining stock affiliated with Beach’s uncle.

In August 2008, while the two men were playing golf in New York, Beach suggested Trump Jr sell his shares in the tanking stock “if you need the tax loss”, according to a copy of his testimony in a long-running civil lawsuit between Beach and a former employer, hedge funder Paul Touradji.

Beach’s father, Gary Beach, was convicted last month of federal bankruptcy fraud after a seven-day trial in Dallas.

Trump Jr testified that he had other business discussions with Beach but not all of them came to fruition, including a plan to buy a hunting preserve in Mexico. Trump Jr also referred Beach to someone he knew from Saudi Arabia when Beach was working on a potential oil purchase and invested $50,000 along with his sister Ivanka in an Argentinian resort developed by one of Beach’s friends, he testified.

Trump Jr estimated he exchanged roughly 500 emails with Beach at the time of his 2010 deposition. Pressed for details about the oil well deal, Trump Jr indicated he was not well versed in the oil-and-gas business.

“You know, I put some money with a friend,” he testified.