How much jail time would you get as a 21 year old male in California on probation who's been in and out of jail a few times and got caught with pot and when asked his name, pretended to be someone else?
They charged him with a Felony 529 PC FALSE PERSONATION
And Misdemeanor (A) 11357(b) H&S POSS. OF MARIJUANA 28.5 GRAMS OR LESS

Depends on what his previous arrests were for. He has (obviously) been charged with a felony this time but other factors will enter into sentencng, particularly the fact that he's on probation. The Court may very well not trust him again and sentence him to complete the original sentence for which he got probation PLUS time for these new charges.

Does he have a good Attorney?

I know a guy that went to jail for the same reason in Pennsylvania. He got 3 years. But you never know, depends on the state he committed the "crime".