Photostream pics disappearing but not found on roll

Hi,

My son has one iphone with one icloud account. I decided to backup his phone to icloud but when i switched that on from inside the iphone, it removed the 900 images he had in photostream. After noticing the photostream album had disappeared, i stopped the icloud sync and it came back with only 18 photos ( i assume from last 30 days). I don't know where to find the missing 900. I also don't understand why he would have pictures in photostream that he took with his camera, but that don't exist in his camera roll. I understand he would have some from other apps and sharing etc but he has lost hundreds of pics he took on that phone that were in photostream but not in roll- Could anyone please let me know how I can recover those images- i've tried some third party apps but they are only brining back the images that I can currently see normally.

Thanks