Can someone be charged with armed robbery with no witness or evidence to prove a crime was committed?

I would need to see the police reports before I could give an opinion on the matter.

Even the most inept prosecutor would never charge someone with a crime with no evidence and no witnesses. There must be something or else they wouldn't have brought charges. There has to be a victim, i.e. witness, in order for armed robbery to have occurred. The statements by the witnesses or victims, whether they are pressing charges or not is evidence and whether they want to appear in court and testify or not, they can be compelled to do so. Also, if he confessed, that is evidence as well. If the gun that was used can be traced back to him, that is evidence as well. Armed Robbery is a very serious offense and he needs an experienced criminal lawyer to represent him.

Most criminals are not very good decision makers. The police will manipulate them into confessing and that takes all the bargaining power away from their attorney. The prosecutor can then demand that you plead to the charge and you can only try to get less than the maximum. The victim does not have the power or the right to "drop the charges" and he must have reported the crime and identified the perpetrator so the prosecutor did have plenty of evidence. He had a witness to the crime and possibly the gun that was used. Your brother committed a very serious crime and if he has a prior felony the sentence will probably be 8 years or more. He could get 15 years for armed robbery and prosecutors do not reduce armed robberies or gun charges. Judges will only give minors a break on such violent crimes, and even minors will get long sentences if they have a record or use a gun. If he had remained silent he might have gotten a better deal or even beat the charge, but he confessed and that is bad for him but good for society as people who commit such violent crimes are very dangerous and the public must be protected. Hopefully he will change his ways and become a productive member of society.

If there was no evidence whatsoever, how can they charge him? It sounds to me like he needs an aggressive attorney on his side, or he will be going to prison.

In NY State court a conviction cannot be based on a confession alone. The DA need some evidence to corroborate that a crime took place. If the DA subpoenas the victim and forces him to testify he was robbed that would be enough.

The confession is all they need. What other evidene do they need?

He need to hire an attorney but his admission is going to be a problem. He needs a retained criminal attorney, 8 years is a very long time in a prison.