If convicted, he is almost certainly facing a prison sentence. However, the circumstances you describe make him not guilty. Presumably he was seen by a police doctor upon arrest, and all his injuries were recorded?
Your brother having no previous is only a factor if he were to be convicted.
Where are all the witnesses backing up his account?
Was the other guy arrested? What are his injuries?
Like I say, the above account makes him not guilty, as he hit the other guy by accident.