As you know, Bam's been on the road with his 'Jackass' costar ... opening for Steve-O on the first leg of his comedy tour. Bam even brought his son Phoenix onstage with him at one of the stops. All seemed to be well -- that is, until Bam posted a shocking caption this week that spurred a hard truth from Steve.

Bam threw up a pic of himself, Johnny Knoxville and SO, writing ... "One arm is a best friend who had my f***ing back all along, another arm is a poser. A pro creation thief. If Jeff tremaine was in this photo, he would be to greedy to fit. F*** jeff. Remember I tattooed that on me long ago jeff? It’s because I knew who you were all along."

He added, "@steveo much love and respect."

Steve-O responded in a since-deleted comment ... which is pretty hard to read. He accuses Bam of staying up all night and getting loaded after the last show -- when he made the post -- and says he's now bracing for the prospect of hearing about Bam's passing, and there is nothing left he can do to save him.

While Steve might've nixed the comment, his feelings haven't changed -- telling TMZ he'll never stop being there for Bam if he wants to actually achieve sobriety. But if not, he says he's done all he can ... and that there's nothing more he can do to try and make him want to recover. He decided to delete the comment after a few minutes and had nothing else to add.

As we've reported ... Bam's been on a very turbulent journey over the past year or so, hopping in and out of rehab facilities -- oftentimes under a court order -- and continuously finding himself in situations where he's partying his face off and drinking to excess.

It's sad. At this point, even his closest allies who remain in his corner are starting to wane.