Generally, 'Ya' means 'already', while 'ahora' means 'now'. Using one or the other just depends in what you want to say. But we have to say that 'ya' has a wider usage with respect to the english term 'already', it emphatizes on how much an event rapidly occurs. For istance, in that sentence, using 'ya', the speaker is tring to emphatize on the fact that listener's son took just a little amount of time to become a lawyer, or maybe just that time passed so fast. While if he would have used 'ahora', the sentence would be just an observation of the fact that listener's son is in this moment a lawyer. Hope you got the idea!