Wifi Greyed Out on iPhone4s After Update to iOS7.0.2

After updating my son's iPhone4s to iOS 7.0.2, his wifi has stopped working. His phone has now jumped onto his 3G cellular connection and when attempting to access wifi setting (Settings > Wi-Fi), it is greyed out.

I've connected his phone to iTunes, backed it up and "Restored iPhone" (to a base empty iPhone4s on iOS7.0.2). When going through all the welcome screens on the iPhone after the restore, I wasn't asked to confirgure Wi-Fi (I believe you usually get asked to select a wi-fi network). I then accessed setting and saw my wi-fi network listed and was able to connect briefly. Over the next several minutes I watched as the icon in the status bar at the top of the iPhone switched back and forth sparatically between the wi-fi symbol and 3G. At the same time all other iOS devices in my house were stable and remained on wi-fi.

1. iPhone4S (iOS 7.0.2) - my son's device EXPERIENCING THIS ISSUE
2. iPhone4S (iOS 7.0.0) - my older daughter's device, WORKING FINE
3. iPod 4th Gen (iOS 6.1.3) - my older daughter's device, WORKING FINE
4. iPod 4th Gen (iOS 6.1.3) - my youngest daughter's device, WORKING FINE
5. iPhone4 (iOS 7.0.0) - my partner's old phone, WORKING FINE
6. iPhone5s (iOS 7.0.2) -my partner's NEW phone, WORKING FINE
7. iPhone4 (iOS 7.0.2) - my old phone (5s on order), WORKING FINE

His iPhone is less than a year old, so I hope this is covered by warranty or that Apple will fix this in an update to iOS7.

Has anyone else experienced this? Any solutions?