The actor, who turned 60 yesterday, said Jett's death was "the worst thing that's ever happened in my life".
Travolta has credited Scientology for helping him through it.
"The truth is, I didn't know if I was going to make it. Life was no longer interesting to me, so it took a lot to get me better," Travolta said in an interview at the Theatre Royal Drury Lane in London, according to the BBC.
Travolta and wife Kelly Preston's eldest son, Jett, died suddenly at the age of 16 in January 2009 from a seizure while on a family vacation in the Bahamas.