(CNN) -- When Alan Gross reaches his 65th birthday next month, he will tell himself that it will be the last one he spends in a Cuban prison cell, Gross' attorney said Wednesday.
"His hunger strike took a toll on him," Gross' attorney Scott Gilbert told CNN during a trip to Havana to visit his client. "He told me yesterday that his birthday that he marks on May 2 would be the last birthday that he marks in Havana."
Gross is serving a 15-year sentence for bringing satellite communications equipment to Cuba as part of his work as a subcontractor for the U.S. Agency for International Development. He was convicted in March 2011.
U.S. officials said Gross was merely trying to help Cubans bypass the island's stringent restrictions on Internet access.
But Cuban authorities say Gross was part of a plot to create "a Cuban spring" and destabilize the island's single-party communist government.
"He was sent to Cuba five times on a government mission and on a government mission that was blatantly illegal under Cuba law," Gilbert said. "He was arrested, he was convicted, he was sentenced to 15 years in confinement, and to date, our government has done virtually nothing to obtain his release."
Earlier this month, Gross embarked on a nine-day hunger strike after reports surfaced that after his arrest, USAID had tried to create "a Cuban Twitter" using cellphone text messaging services on the island.
Cuban officials responded angrily, saying the text-messaging program showed that Washington was still trying to subvert the island's government.
While USAID and State Department officials defended the now-inactive program as a way to reach the Cuban people, U.S. Sen. Patrick Leahy said it had put Gross at greater risk.
Gross later said he was suspending his hunger strike after his mother pleaded him to begin eating again.
Since his arrest, Gross has suffered a slew of health problems, his attorney said, and lost over 100 pounds.
"Alan is confined to one room, 23 hours a day, he spends his day there in pajamas, he's fed meals in his room. He's let out for an hour a day, to exercise an hour a day in a small, walled courtyard where you can barely see the sky."
Cuban officials have said they want to negotiate Gross' case with representatives of the United States in conjunction with the fates of three Cuban intelligence operatives serving lengthy sentences in U.S. prisons.
But U.S. officials have called that effort blackmail and said the cases are separate matters because Gross was not working as a spy in Cuba.
The diplomatic impasse is taking its toll on Gross, Gilbert said.
"The White House has yet to engage on this issue; we need the president to make this a priority. Without that, Alan will die in Cuba," Gilbert said.