Now, I can see a Sim getting a wish that they can't actually complete...the game does weird things sometimes. However, the fact that he can actually take plasma packs from the fridge suggests that he has the "Vampire" hidden trait. That's one of the abilities that it gives. Does he have any other Vampire only interactions? I.E. Can he consume the plasma packs without becoming ill? Can he hunt or drink plasma directly from other Sims? Does he smolder in sunlight?
When he drinks plasma packs, he doesn't get the nauseous moodlet, but he does vomit some time later. He smolders in the sunlight, but can't hunt, and as far as I saw, he couldn't try to drink from another sim. He still had the option (albeit greyed out) to ask his father to turn him into a vampire. Seems like he got the downsides to being a vampire without much in the way of benefits. I didn't check his skilling speed; though, as toddlers, I know at least
one of that set of triplets was learning xylophone at an alarming rate.
And I've occasionally seen him very briefly use the genie run animation (purple cloud) before his vampire run kicks in. It's like the game got confused with him!