The 'Tiger King' star's attorney, Francisco Hernandez, tells TMZ ... Joe is distraught after getting a phone call in prison from Dillon saying he's planning to leave him.

We're told Joe felt like his stomach got pulled out of his body when Dillon broke the news -- and now we're told Exotic's considering beating him to court, and filing a divorce petition before Dillon can.

Dillon says the split wasn't an easy decision -- but adds he'll continue to support Joe while he's in prison. He says they are still on good terms and hopes it stays that way ... although it doesn't sound like Joe would second that emotion.

As you know ... Joe and Dillon got hitched back on Dec. 11, 2017, tying the knot 2 months after Joe's previous husband, Travis Maldonado, shot himself in a fatal accident.

We've learned that after the emotional phone call, Joe fired off an email to a friend ... saying he feels abandoned and alone and just wanted to mean something to someone. The way Joe explained it, Dillon's getting lonely with Joe locked up and wants a boyfriend.

We're told Joe is undecided on whether he'll file for divorce -- he's got a lot on his plate just trying to gain his freedom -- but of course, Dillon could pull the trigger first.