My point is that no court is going to believe that a pencil just happened to find it's way from your BFs pocket (which pocket by the way ?) into the victims eye lid.
In my view there is evidence of 'intent' from 2 points, first the fact that your BF is a boxer and as you admit he knows that using his boxing skills can count as use of a weapon. Secondly, that your BF admits to having a pencil in his pocket and that after the fight the same pencil was in the victims eye.
That is why I am asking which pocket the pencil was in , for example it is just about conceivable that just by chance, a pencil that was in a top pocket of a jacket, ended in the eye of the other person in a fight. I can think of no way that a pencil that was in a back, side or inside pocket could end up in the victims eye just by chance, it would need to have been taken out of the pocket to be used as a weapon. This would be proof of 'intent' and justify a charge under S18, GBH with intent.
A section 18 offence is virtually certain to be a jail term.
Your BF needs to get all his facts in order and get a good solicitor urgently , there IS a possibility of a prison sentence here.