I'm looking into trying to rehome him now... but who wants a rooster like that ?! He's not even full grown :/. Worst thing , my son hatched him as a project at school and loves him

maybe a rescue place ?
I moved our little girl inside. She's moved a little, but won't take water at all , even with a syringe. I'm scared to be too rough :/
The only people who want a rooster like that want him for the table. You *may* have luck rehoming him, it would be irresponsible not to disclose the reason you seek to rehome him.
...and were it my child, they would be getting a lesson in the responsibilities that come of having pets.
/cut and pasted from one of my other posts -
"When the need arises - and it does - you must be able to shoot your own dog. Don't farm it out - that doesn't make it nicer, it makes it worse." - Robert A. Heinlein
I don't expect a chicken to act like a human, but i DO expect it to know its place in the flock. It is no kindness to allow an aggressor to live and continue abusing the innocent (or yourself) for reasons of emotion. Nor do I think it appropriate to "rehome" a rooster to someone unaware of his aggressive behaviors. As an NPIP certified breeder, I take flock security very seriously - poultry leaves my property, it doesn't re-enter.
Aggressive Roosters become meals for myself and my family.
I take no joy in it, but neither do I shirk from the duty. The bird has chosen its place in the culling line by its behaviors, I merely give motive force to its election.
/end cut-paste
It may be possible, as that's a juvenile rooster, that a flock of older hens could set him straight. Maybe. I have no shortage of roosters, and always one ready to replace my current breeder(s) - so its not an opportunity I'd explore.
Otherwise, you've recieved good advice. Separation from the flock, clean the wound, triple acting ointment, attempt to keep hydrated and the wound from secondary infection. I've had chickens survive pretty horrific wounds. If she stays hydrated, and starts eating, that's a "maybe" in my opinion.