Does he like the 3GS? If so, tell him he should pre-order a 4S. It's going to please him to no end. Android just isn't going to cut it unless he is already dissatisfied with iPhone/iOS.

In the meantime, he can update that 3GS to iOS 5 to see if he likes the new additions/changes to the OS. I'm personally waiting for an untethered jailbreak before I press the update button.