But the Demon ranking is not about strenght...
Going through most of the series, Demon ranking seem to be based on the following:
- The Demon's Strength
- The Demon's Influence throughout the Netherworld
- The Demon's Achievements throughout the Netherworld
And Demons of High Rank are usually expected to possess a certain level of strength, a reason why we see demons being at awe of other demons of higher rank, or even how demons of high rank having pride in their rank (for example, Kalego has pride in both his and Balam's rank as 8 ranked demons). Not to mention if there's a demon that's low in rank, but they possess an impressive amount of power, it becomes questioned as to why that Demon's rank isn't higher (we saw this when Azz was fighting Ifrit, and Ifrit was actually upset that Azz's rank wasn't higher to properly match the power he displayed).
Now as for Iruma, I'd say he still lacking in terms of strength and a little in terms of influence. Most of Iruma's influence (outside of his inner circle such as his connections to waterpark, idol work, bachiko, as well as his classmates, etc) stems from simply being the grandson of Sullivan, so it's less Iruma's influence and more of his Grandfather's. As for his strength, while Iruma is capable of performing high level magic and has access to large amounts of magic power, it's usually due to the help of his ring that has Sullivan's magic stored in it for him. Individually speaking, Iruma is still learning things such as non-verbal magic for different spells.
Now when it comes to achievements, Iruma has definitely accomplished enough where he'd at least complete that criteria. So all in all, that's why I think it's too soon for Iruma to rank up, at least too soon to go up to rank 7. Of course, someone could argue that Iruma's a unique case since Ali's strength is dependent on Iruma's demon rank, and Iruma ranking up would increase Ali's strength and in turn Iruma's, and in which case I wouldn't disagree.
Sorry for the rambling.