Passport Validity for UK Travel

Hi,

My son's Indian passport will expire on September 17th 2015 and dependent visa will expire on Jan 31st 2016. My son now in India for vacation and planning to back to UK on May 2015. so my questions are

1) Is there is any requirement for passport validity (minimum 6 month) for UK travel even has valid Visa?..Because at the time of travel he will have 4 month validity but valid visa upto 2016 jan

2) Is there is any passport validity issues while bringing my son on May 2015?

3) Minimum 6 month Passport validity is only applicable for applying visa?

Kindly help me on this. I need this very urgent

Thanks
boraj