passport validity before applying for ILR for child in india

[shez80]

We as parents have got ILR. Our son is 5 years old, he was born in UK, stayed with us for 1.5 years and then stayed in india with grand parents for 3.5 years. we are trying to bring him back here, so tried applying for his visa as his visa is expired already while he was in india. his passport is expiring in 4 months , do we have t renew his passport before applying for his visa or they can issue him visa in this passport and we can renew it when he comes here in london as his passport was issued a london. please advise how how much validity of passport is needed before applying for my son's ILR and would he be able to travel here to get the new passport done here. his passport is expiring in jan 2013.