Elijah Blue Allman was stone-faced heading into his temporary appointment hearing Monday -- where a judge will presumably decide if Cher will, in fact, be tapped to head up a conservatorship for her adult son ... this after temporarily denying her earlier this month.

In early Jan., the court blocked Cher's effort to put Elijah into a conservatorship -- siding with Elijah's attorneys when they argued they hadn't been given enough time to make their case against the conservatorship ... plus, not getting docs they needed to combat her request.

Cher cited her son's ongoing battle with addiction as the key reason why she wants the legal apparatus used on him -- claiming he can't be trusted to manage his finances on his own.

She specifically says that regular payments he receives from a trust fund left behind from his late father, Gregg Allman, are likely to be squandered because of issues she claims Elijah continues to experience ... but he's called BS on that, saying he's cleaned up his act.

Welp, the judge heard the arguments anew ... and he once again shot down Cher and her team -- simply saying they hadn't proven Elijah doesn't have the capacity to handle his own affairs ... going on to say that an alleged bipolar disorder diagnosis, in and of itself, doesn't necessarily mean he's incompetent. As the judge said, that doesn't address capacity.

The court also acknowledged that Elijah has proven in paperwork that he's remained clean and sober of late ... plus, he's also proven that he's handling his money just fine.

The judge set a future date for a trial on this issue -- where it will presumably be decided once and for all -- and went on to suggest Cher and her kid head into mediation, STAT. Cher beamed in remotely BTW ... it was only Elijah, his wife and all the lawyers in court today.

Our photog got Elijah and his lawyers on the way out as well ... and they had some words.