Well given that BOTH his parents are also skeletons I tend to think he was simply BORN a skeleton illogical as it seems (hey if the original orgin story for Casper was he was born a ghost, this isn't all that different). The bigger issue is Jr's ability to flip back and forth from Skeleton to fleshy. Granted he only got that power after he got Nergalized, but since the functional power of Encroaching Doom Syndrome is turning a dead individual into a living one, I'm not sure how significant it would be to him. If he can get it I tend to think the Nergal symbiote can functionally "cure" him.
And I think Jr must have at least some of the Grim Family DNA since he has some of grims odd traits, like the ability to see with no eyeballs (though he apparently has enough leftover humanity that having one doesn't blind him on that side, like it does grim.)
"For I have heard the Song of War, and to me, it sounds just like "Please, Mister Custer.""
Cogito Ergot Sum, "I think there is something wrong with this rye bread." (Aplogies to Seafrol.)