Right, I've noticed some pictures where it will recognize multiple people, but I have many pictures of me holding my son as a newborn, for example, and it only recognizes me.
Face detection is a very challenging task. Apple improved the face detection vastly, but it simply still not as good as the human brain in recognizing faces in a photo. It is amazing that such a complicated algorithm can run on an iPhone at all.
I was hoping I'd be able to add him so that the photo would display in both of our albums under the "people" tab.
You can do it on Mac, but not yet on an iPhone.
had an album on my previous phone, which I created within the photos app, for all the photos of my son. When I setup up my new phone using my backup, that album is gone. If I search for it under "albums" I can find it, but that seems to be the only way to access it.
If you can find all photos of your son this way, select them and create the album again from the selection.