The good news is that the HK visa office is very professional and from my experience, very efficient. A request for photos or other documents is definitely a good sign that they are working on the case. Since they only requested an additional photo, I'd take it as good news. If they were going to request more important items (relationship proof or IMM forms), they would have sent those requests at the same time they asked for the new photo. So, it looks like everything else is in order and it's just a matter of time before e-cas is updated and the PPR is sent out.
I wouldn't be worried at all. I think your son will be finished by the end of this year, if not sooner.
For our case, I think it was a combination of things that worked out in our favour. I also think that we got lucky and our case was reviewed by a highly competent visa officer.
Good luck!
FS