If it is really just for an emergency situation its pretty easy. Just monitor the transactions and if there are any, there was an emergency or he was abusing it.

Teaching your kids to use credit responsibly is one of the best things you can do for them. Credit card companies are very aggressively targeting college kids, and they are pretty successful at convincing them to sign up for cards. College students have been known to run up massive debts that they have no idea how to control, and are often too embarrassed or scared to tell their parents, thus beginning their adults lives in the red and with a terrible credit score. Watch the documentary "Maxed Out" for some real horror stories, including kids committing suicide over their debt.

I strongly encourage you to give him an emergency card, that is only for real emergencies, and also a low-limit card that you teach him how to use. If he charges only what he can pay off every month, and learns to do that, he will develop great credit and great spending habits. Credit cards should not be the great unknown, or the alluring taboo, but rather understood as a financial tool with real risks and real rewards. Help him out by being his teacher, who accepts his mistakes but helps him understand how truly important his credit is.

Try using a pre-paid card for a time, with a pre-loaded limit on it. The balances can usually be checked online, and you can load money on the card using a bank transfer, and some will even link to PayPal accounts.

If you do this, then he has it for minor emergencies with a small pre-loaded amount (maybe 200-500 bucks?) and then you can always transfer money onto the card for him for a bigger emergency.

Once he gets the hang of this, doesn't abuse it, doesn't spend it when he gets a little 'tight' and wants a pizza or something, THEN you can consider getting a credit card with a balance and a limit and all that jazz, and he can start building credit.

To build on michelleldevon's comments, I'd suggest that your son sign up for a pre-paid card now using his own money. It's no different than spending cash, yet offers the convenience of a card while allowing him to build his credit. Capital One gives these to literally anyone. Assuming he still lives at home during the summer months, the bills will be sent to your address and you can check them when they arrive.