Someone once mistakenly took my all too generic black laptop backpack on arrival at GLA from LHR a few years ago. I realised as soon as I stood up, there could only have been a handful of pax in front of me. A quick word with the FAs, they suggested to hot foot it up the looong corridor at GLA to see if I could catch up with the culprit, or just go to lost luggage. No sign of anyone or my bag, I resigned myself to reporting it lost (or possibly stolen) at the baggage counter in reclaim; but to my astonishment the chap already knew what I was on about, and two mins later it appeared on the carrousel ! No idea who took it, or how it managed to find its way onto the carrousel, but all contents were still there.
I now have some bright identifiers on my bag, and no, not BA tags ! Honest mistakes do happen and I feel for the OP and their son, it feels absolutely rotten when it happens