Darian Dalili, son of Shahab Dalili, a US permanent resident who has been detained in Iran since 2016, sat outside the State Department in Washington on Tuesday, surrounded by signs demanding his father's release.
Both father and son are on their third day of a hunger strike.
Last week, Iran released four American citizens from Evin prison and placed them under house arrest. A fifth American had previously been placed on house arrest.
The elder Mr Dalili was not among them.
“When news broke out on Thursday that there's a deal that is being done and my dad is not in it, that was the trigger for me,” Mr Dalili, 28, who drove to Washington from Massachusetts where he works as an engineer, told The National.
“First I went to the White House, where I began my protest with a sit-in, and the second day, I began my hunger strike at the same time my father began his hunger strike in Evin [prison].
“He tried to persuade me not to do it.”
The elder Mr Dalili, 60, is a retired shipping captain who was detained in Tehran while attending his father's funeral. He was handed a 10-year prison sentence for aiding and abetting a foreign country – charges he has denied.
At the weekend, Iranian state media reported that the prisoners would be released in exchange for the unfreezing of $6 billion in state funds held in South Korea.
The US government said the deal does not involve a change in policy towards Iran, which is the target of a number of western sanctions.
One of the signs near the sit-in site reads: “Please include Shahab Dalili in Iran deal $6B.”
Mr Dalili has never been recognised by the US government as having been wrongfully detained, a crucial designation for inclusion in any prisoner deal or official effort to negotiate his release.
“The five Americans who have been moved from prison to home detention and expect to come home are Americans who have been found to be designated as wrongfully detained,” US Secretary of State Antony Blinken told reporters in response to a question about Mr Dalili.
“We continue to look and will always continue to look at the situations, conditions of other Americans around the world.
“As a matter of policy, we're constantly reviewing whether any particular individual whether an American citizen or legal permanent resident who is incarcerated in another country is unlawfully detained,” he added, “and that, of course, triggers a whole series of actions and steps that we take to try to secure their release.”
Mr Dalili said the US government has had seven years to investigate his father's case and he wonders if the goal was simply to allow his sentence to elapse.
Still, Mr Dalili, who spoke to his father on Monday, said with the deal for the other prisoners still in progress, he remains hopeful his father could be included.
He has been sleeping in his car and said he wants to continue with his protest, though does not know how long he will last.
“My mind is slow; it takes me a while to think before I can answer,” he said of the effects of the hunger strike.
“I have these hunger pangs in my stomach, which are normal, I guess. Otherwise, I have a very distinct feeling of exhaustion.”
He expressed hope the State Department will include his father in the new deal, as seven years is “way too long”.
“I, personally, as his son, have no reason to believe that the charges made against him are in any way true,” he said. “But even if they were, shouldn't the onus be on the US government to bring him back?”