Basically there has been a common assault (which is now arrestable since the end of last year) but if he has lost movement in his arm then there could be ABH or even a GBH if he has a permanent injury. HOWEVER, no self-respecting Detective is going to take this matter as anything other than a squabble unless you and he fit one of the following:

1. He is a minority person and alleges racial/homophobic etc.
2. He is an asylum seeker.
3. You are a police officer.
4. He is a pupil and you are his school teacher!

Definately do not give anything to the Police. If however, you are contacted by the police regarding an allegation then make your counter allegation of assault. The reason you didn't report it sooner being that you took it for what it was - a minor scuffle and you didn't want to waste the time of the police.

Your local nick will deal with loads of these petty allegations every day from time-wasting tossers that just want to use the Police as a way of getting back at someone. These usually get dealt with by means of a telephone call to BOTH parties telling both to grow up. I wouldn't worry too much.