First of all, he needs to not make any statements to the police or Prosecutors. If he has already, what's done is done... but he should not say anything else. Its not unusual for the police to recontact a defendant for "clarification" of their first statement... but in fact what happens is they confuse the person or the person doesnt express themselves the way they intended and the cops then claim the defendant has given an inconsistent statement and therefore is lying and guilty.
He needs to contact a lawyer because every case is different and defenses are different. If they used an undercover cop it makes it a little more difficult as their credibility will be somewhat high. He needs to write down exactly what he remembers happening and what was said to him by the cop. If he can't afford a private lawyer he should contact the public defender who are also lawyers and often very experienced in their particular court. There are often lesser charges that could be negotiated (disorderly conduct for example, which sounds a LOT better than solictation of prostitution) or situations in which the case could be dismissed upon taking education classes or waiting for a period of time with no further incidents. It depends on the court system and the luck of the draw as to which prosecutor you get. Hope this helps!
The most important thing he can do is contact an attorney immediately. Generally this is a misdemeanor punishable by up to 90 days in jail and a $1000 fine. It's important that he not discuss the case until he hires a lawyer. If he has no prior history, it's likely that a lawyer can work out a favorable plea bargain. You can call our office for a free consultation!
You did not mention what state this happened to your brother in. It is lawful for the police to conduct this type of sting operation.
Only an experienced local criminal defense lawyer in your brother's community can evaluate the case and determine if a defense exists. If he cannot afford an attorney, he should request a public defender to be appointed when he gets to court.