I am assuming that your son is fresh out of high school.
Law degrees in the UK are three years and are an undergraduate degree. To continue on to practice in the UK, you have a year of what is called the LPC. You then do two years as a trainee before being fully qualified.
Now, to the best of my knowledge: if he wanted to pursue a career in law in the US, he would be the same as any other student who had completed an undergraduate degree and would have to go to law school. HOWEVER, I have seen adverts in the backs of some magazines here offering a 6 month course to study for the New York bar. I can't speak for the accuracy or reliability of those.
I am studying law here in the UK (in Manchester) but I don't plan on trying to practice in the US so I'm not very knowledgable about it. I would have a look at the American Bar Assn's website or write them and they may be able to help you.
good luck!