I have three theories: (Personally i feel like #3 would be the most interesting. #1 would be kinda anticlimactic, #2 seems like a bit of an asspull)
1) eventually Iruma's body will adapt to be able to use mana as his own so even though he is human he will have demon abilities.
2) he'll get injured and get a blood transplant that mutates his cells.
3) we will find out that humans actually are able to use healing/light/life magic (that's why iruma's blood heals wounds and accelerated that beanstalks growth ) and its probably an ability that doesn't manifest outside of the demon world so humans are believed to be manaless. There might be a human hiding in the demon world who will teach iruma to use his human magic.