Accidentally hit post 
3: Headbutting is vile, vicious, and one of the more extreme uses of physical violence. At best it's a caution for assault but if injury caused would likely be ABH tho impossible to know without further details and at worst it could be manslaughter as headbutts can lead to someone going down and smacking their head. It can be that simple - one punch or one headbutt can result in death and then a manslaughter charge.
4: I honestly appreciate you are in a very shitty position and that's it hard but when you strip it down the view of your position becomes easier:
- At 18 he is criminally responsible as an adult;
- You enabling him will not help him in any way - he needs both accountability and help;
5: An act of such sheer violence as headbutting rarely comes out of the blue, does he have a record and maybe already on a caution and that's why you/OH have not reported him to police?
6: The likelihood of him re-offending again is very, very high; can you see that - IE can you see headbutting is not some minor issue but a huge red flag? If you can grasp that simple fact then you'll see that reporting him now will likely help him in the long term. No consequences or intervention now will, unequivocally, not 'help' him.
7: What are OH's injuries?
8: Are there other minors in the house?
9: What's the context of this? There must be a background/backstory if he's getting paralytic at 18 and headbutting someone (& no, it makes zip difference that he and OH are both males and he's the younger of them; this is DV and, at risk of repeating myself, he will re-offend without consequences and interventions).
10: How would/will you feel when he does that to a GF or a random member of public? As that response is the same one - no matter how hard it is for you or how much you love him - that should be applied now.
Please report him, for his own sake as much if not more than anyone else's TBH.