Hi OP, I'm really sorry to hear that whether intentionally or unintentionally, your eldest son has been left out of the class photo.
This won't address the issues, but may I suggest a quick DIY fix? This is what I would probably do...
There's a lot of programs on the computer that are free and relatively easy to use (removebg, canva) but of course if you are skilled with other programs or know someone who is, feel free to use one of them... I would get a photo of your son, remove the background, open up canva, take the scanned photo of the class (if you have one, when I went to elementary school a million years ago, if you happen to miss class photos, you still got a class photo, maybe they don't do that anymore)... Then add your son into the photo. You can adjust the layers, or even play with the photo a little bit, editing brightness and saturation etc of your son to match the class photo.
You can take a drawing that your son has done, and use it as the background, or incorporated into part of the design.
And then you can get that printed and put it on your fridge or framed or wherever you would put it, to make him feel included (if you think that would be beneficial.)
When I was a kid, a couple of times in elementary school for one reason or another I missed out on class photos, I don't recall if I felt sad about that or not.
I think if I were in school today, that I would like to have the photo digitally manipulated to add myself in.
I actually used the above method that I briefly described here, to add myself to a group photo that I literally took just the other day. An organization that I belong to, but I was the photographer, and I added myself into the photo after the fact. Most people didn't even realize that I wasn't in the photo (even though I was the one taking it,) and a few that did see, commented "hey magician , I'm so glad you were able to add yourself to the group photo!!" :)
Hope this helps...