Well, if it were me, I'd conclude that, however old at the time, they needed someone to imprint on and they chose your son. Did you know that in Chinese culture ducks are the symbol of fidelity because of the strong bonds they make and their everlasting loyalty to them?
I hope he revels in the attention. Ducks are so affectionate and funny that I would imagine he does. I'm smiling ear-to-ear in my family's memory and in anticipation for your family. I hope you guys always will too!
Can your son hand feed them? When we became holiday parents to my daughter's kindergarten ducks they loved cucumbers and my kids could hold one end and the male and female would work the other end furiously. If I got long English-style cukes the kids were confident enough about hanging on through the feeding frenzy. If you try it, cut off the end so the ducks can get to the soft core right away and pull less against your son's grip.
I also have a happy memory of the kids swimming in the pool with the ducks. But, Oh Mama!, what they did to the water! We had the only emerald green water in SoCal! (Naturally, at that point the kids weren't in the water with them anymore! Thank god for our pool guy!!!!!)