If you want to stop it, take action soon. It's easier to
stop loss as opposed to
regrowth. Rogaine can help, and it does work, especially in situations like this.
Or, as dlish suggested, check out propecia. Be warned, it is expensive and a lot of insurance plans won't pay for it since it's considered a "cosmetic drug". If that's the case you can talk to your doctor about possibly prescribing Proscar (it's a prostate med, but it's the same ingredient, just 5x as much) and chopping up the tablets. Cutting one Proscar into 5 roughly equal pieces is tough, but I've seen some determined guys do it. Just don't ask your pharmacist to do it for you.