Reevoo
Sooo, whats the benefits of bonding with a phoenix? he's also young so his magic haven't matured yet, which should give him some special benefits later on or even now, because of their magical bond. Can they communicate right now or must the Phoenix be older, even if they can't communicate they should still be able to understand it, right? How do you take care of a phoenix Anyway? Wait i just remembered that it's a special Blue phoenix, what can it do? Can mc and the phoenix mutate or upgrade/making their magic purer and/or more powerful by their magic bond? Also dosen't mc have much more magic than a Child of his age? Why dose it seem like he dosen't? Maybe a part of his magic is fighting against the deciece/illness so he's not at his maximum. Can he sometime crate his own spells or maybe modify or/and upgrade the already exsisting ones. Last but absolutely not least, there are probably more hidden rooms or Places in hogwarts that noone could find. Maybe it wasn't only salazar slytherin that made a secret room, but the other founders as well. If there was can he find them, loot them and play in them too! Thank you for your time you spent reading this comment. . . . . Love the book btw.
For the MC's illness. Consider a ritual involving Fitz. A rebirth ritual that will purify his blood. "Cloak of a dementor, purest of vile, my life will be given" "Blood of a unicorn, willingly given, my blood will be cleansed." "Ashes of a phoenix, first of the many, I will be reborn" Something to think about. :)