It depends. With a Section 20 GBH (without intent) if it's not serious enough to be sent to the Crown Court, and he's tried or pleads at Magistrates' Court the maximum sentence is either an unlimited fine and/or 6 months' custody.
If it goes to Crown Court it's up to five years' custody. I don't think he's got any previous so it's likely any custodial sentence would be suspended.
Not good though.