he can try to find a cheaper shuttle service.
some could cost as cheap as $50-60 and you would be the only passenger. you'll need to ask them for a flat rate before making an appointment.
he can also make an appointment for his way back home and they'll pick you up at the airport.
taking public transportation is not very convenient, especially during night time.
by the way, does he go to church's college fellowship? usually they like to help each other out. there's a cantonese church in Mountain View.
You can try Super Shuttle (the blue van) which is available in many cities over the nation. My family members used it all the times when they travelled to SF. It's a door-to-door service. The van driver will come to his apartment and ring his door bell. The only inconvenience is that he may need to leave early as the van usually have to drop at different places to pick up more passengers.
You can make reservation online, the website is www.supershuttle.com For Mountain View, the cost for trip to SFO airport is $90 for exclusive van service (private van up to 9 passengers) or $85 for ExecuCar Sedan Service (up to 4 passengers) or $31 for the first passenger and $10 each in extra for Shared Ride Van Service (up to 9 passengers in party). If your son can find a friend who is willing to take the van with him, it's only $41 which is less than half of $90, then pay his friend the cost of takin BART and CalTrain home which is $8.25 as Mrschao said. It's still less than $50. Of course, he needs to find a friend or classmate who is willing to help. You can see the quote that I run for test below.
My sister had paid $27+$20 for her family (2 adults + 1 kid) for a trip from the SFO airport to my home in SF last year. I would suggest the van rather than the sedan as van is more spacious.