Between the bruises and wounds all over his body ("It hit me, but didn't hurt!"),
the only eating one meal a day maximum ("I can eat twice a day?!"),
very few games/toys, no friends whatsoever,
and being utterly terrified of being abandoned by anyone ever again,
I'd say it was 100% chance he was being regularly abused/neglected by his parents absolute scum before he was adopted by the rain woman youkai.