Minor in Possession: California Minor in Possession

My question involves criminal law for the state of: California

My son, who is 20, was arrested for being a minor in possession of alcohol. I understand that if convicted, the court may (or must?) suspend his driver's license for a year.

It seems to me that the arresting officer did not have any probable cause to stop him and his friends (there were 3 of them) who were simply walking down the street at 9:20 PM. The alcohol was not in public view (it was either in paper bags or in their backpacks, I'm not sure at this point). When they got to the police station, another cop asked the arresting officer what the charges were and the reply he gave was "pissing me off."

Questions:
1) Would holding paper bags be sufficient cause for the cop to stop them?

2) Is the cop's admission of being "pissed off," coupled with the fact that they had not (as yet) been drinking and were simply walking down the street, something we can use to fight the issue of probable cause for the arrest?

3) The section of the law listed on the citation is 25662(a)B&P. As my son is 20, will there be the likelihood of a deal wherein he can plead to either an infraction or a lesser offense (such as disturbing the peace section 415(2)?

Any other advice will be greatly appreciated. Thanks.