I'm more of the 3rd theory, the previous demon king was a human with abilities. Because when the ring that Iruma got was first introduced, it mmediately made me think of the Ring of Solomon, which was said to control/seal demons, though in here it just absorbs powers from demons - which in a way does "control" a demon, because Iruma now controls their power as his own. And in regards to Solomon, lots of books like to use him as s human that has contracts with numerous demons. So, I wouldn't be surprised if the story went that way too.
I also agree with the theory it's kinda sus the grampa just broke a law to get him and iruma is very similar to the demon king
Okay first I would like to state that there is no way Iruma would be able to eat THAT MUCH if he were fully human, so I have 3 theories:
1. he's either part human, the reincarnation of the previous demon king, or his family has some kind of history with demons. Maybe the previous demon king is actually his dad or something, they look similar enough.
2. he's slowly becoming more demonic because of the run-off from his ring. I mean, if using enough high-grade spells caused a ring to develop a demon, I don't see why it wouldn't also cause Iruma to become more demon-like.
Both of these are based on the fact that I just don't think Iruma is fully human. His grandpa wouldn't be so determined to make him the demon king otherwise... UNLESS:
3. the previous demon king was ALSO human... or maybe just something else entirely?