A 'not guilty' plea, on th grounds of 'self defence' would be unlikely to succeed unless it could be shown that the
only way for your son to avoid further attack was by using force (and that he used the minimum amount of force necessary). If he could simply have run away then 'self defence' is not a valid defence in law. It would probably be more sensible to plead guilty (but with mitigating circumstances). However the solicitor will have more detailed information, so your son should follow his advice.
Any young person who pleads guilty to a first offence (unless it is a serious offence, such as murder) will be made the subject of a Referral Order, which can last for between 3 and 12 months. See here for further information:
http://www.surreycc.gov.uk/people-and-community/surrey-youth-support-service/what-is-a-referral-order