Trace Cyrus says his dad Billy Ray Cyrus is ready to fight him in court ... claiming his old man's threatening legal action -- while simultaneously begging him to seek treatment.

The adopted son of BRC posted to Instagram Saturday ... claiming his recent social media post about his dad's health came from a place of love -- adding he could've blown the lid on a whole lot more about Billy but chose not to.

But, Trace claims his father didn't take the message as one of love ... claiming he's now facing legal action from Billy -- calling the move a "disgrace."

Trace takes it a step further too ... claiming "Pappy" -- presumably his grandfather, Billy Ray's dad -- would be so disappointed in him.

And, Trace adds, he still loves his dad ... but, he doesn't respect him anymore.

TC ends with a strong message to his father ... claiming everyone around him might be afraid to tell it as it is -- but, he's not. And, he thinks Billy needs to go get help.

As we told you ... after Billy's disastrous performance at a Donald Trump inauguration event, Trace said his dad wasn't healthy and he needed to get help.

It's caused a serious amount of Cyrus infighting this week ... with Trace's bro Braison working with Billy on an album -- and Trace implied Braison is using drugs and alcohol too.

We've reached out to Billy about Trace's comments ... so far, no word back.