Obviously I won't be able to tell you what, if anything, he was involved in. But there was a great deal of U.S. activity in Nicaragua in that time period, so I'll run down the basic information for you.

In 1979, the Sandinista National Liberation Front (FSLN) overthrew the government of Anastasio Somoza in their socialist revolution, taking over Nicaragua. They would rule Nicaragua until 1990, first through a revolutionary junta, then through a government produced by 1984 elections internationally recognized as free and fair. Much like their Cuban allies, the Sandinistas dedicated extensive effort to land reform, workers' and women's rights, literacy, and medical care, seizing the property of wealthy Somoza supporters in the pursuit of economic equality. The United States was hostile to their government throughout the entirety of the period, due to their socialist policies and their ties to Cuba.

The United States was the driving force behind training, funding, and arming the Contras, a counterrevolutionary ("contras" is shortened from contrarrevolucionarios) paramilitary group that violently opposed the Sandinistas throughout the entire period of their 1979-1990 rule. Some of the most famous incidents of U.S. support include the mining of Nicaragua's harbors and the Iran-Contra affair. In 1984, the International Court of Justice found on behalf of the Sandinista government in Nicaragua v. United States, for U.S. actions violating Nicaragua's sovereignty. Given its power of veto on the UN Security Council, the only body which could have enforced the ruling, the U.S. never paid the due compensation.

The conflict came to an end in 1990, when the Sandinistas lost an election following a campaign of terror and violence by the Contras, who supported the opposition candidate. The involvement of the Contras and widespread accusations of intimidation caused contention over the legitimacy of the election, though the FSLN continued to participate in elections, eventually reelecting Daniel Ortega president in 2006 on a moderated platform of social democracy as a part of Latin America's 21st century "Pink Tide".

If your neighbor was a part of U.S. operations in Nicaragua in 1984, in the kind of situation that could get him shot, it's almost certain that it was in some way related to the Contras. Again, since you have no further information (or actual confirmation that he's telling the truth), it's hard to say more. The CIA was very involved, training the Contras in tactics of terror and violence targeted at civilians, such as through use of the Psychological Operations in Guerrilla Warfare manual, right around that time. However, there were a number of operations at the time, and I wouldn't make any assumptions about your neighbor unless he tells you more.

I find it very unlikely that someone involved in a covert operation would give you teasing specific details. I think he's being vague because he's lying or delusional. Either it's a secret or it isn't. There was lots of US activity in Nicaragua during the 1980s and lots of it is detailed in books, websites, documentaries etc. Anyone who's read a conspiracy website or read around the history of the US military or US intelligence of 20th century South and Central American history knows that the CIA were in Nicaragua in 1984, it's not exactly a secret.

I think if he was genuine he'd either be part of a declassified or already reported operation, in which case he could give you specific details, or he would be part of a still classified operation in which case he would not even mention the fact to you and would have a full cover story for his injuries.

Giving you a bit of vague info just screams 'liar' to me.