Restoring data from lost phone

I lost my phone on 03/01/2015 but at time i thought maybe my son had misplaced it somewhere while he went abroad. He is now back and its now obvious the phone is lost. I am pretty sure i had it backed up to Google. I have now got a replacement phone and installed whatsapp on same number i had on lost phone but the old chats are gone and it seems to have backed up some and not all the phtos. Please help

I need the whatsapp chats urgently. How can i recover them