One of Juan's sons, Ivan Aguilera, who actually served as his dad's manager for the last 8 years of Juan's life, tells us the claim -- made by a man named Joaquin Munoz that Juan faked his death and is alive and well -- is complete BS. Ivan tells us squarely ... his dad is deceased.

He held his father's cremated remains at a memorial service back in 2016.

Munoz claimed to be Juan's ex-manager and told a Mexican news outlet last week the Latin singer was alive at home and had faked his sudden death. That story was picked up by other outlets ... and, suddenly, the buzz was Juan was going to surface by the end of 2018.

Juan's former attorney and rep for his estate, Guillermo Pous, backs Ivan's account ... telling us Munoz never managed Juan. He was just a pal.

Pous thinks Munoz is only peddling this conspiracy theory to sell a new book. Needless to say, Juan's family is pissed, and Pous says they're considering legal action.

FWIW, we obtained Juan Gabriel's death certificate back in 2016. Juan's official cause of death was arteriosclerotic cardiovascular disease.