From your description, it's possible that the charge might only be ABH, rather than GBH.
See paragraph 46 here for a description of the type of injuries which constitute ABH:
http://www.cps.gov.uk...he_person/#P189_14382
Then scroll down to paragraph 55 for a description of GBH injuries:
http://www.cps.gov.uk...he_person/#P189_14382
If your son's injuries were serious enough to constitute GBH, the police and CPS would have to consider whether 'with intent' can be added to the charge. Scroll down to paragraph 65 to see when that would be likely.
The maximum period of imprisonment for both ABH and GBH (without inyent) is 5 years. The maximum period of imprisonment for GBH with intent is life.
The document which judges must refer to, when considering sentence, is here:
http://sentencingcoun...ine_-_Crown_Court.pdf
It's also possible that the court could pass an 'IPP'. (An 'Indeterminate Public Protection Sentence'). That's broadly similar to a life sentence, in that a tariff term is set by the judge but the prisoner can't be released (even when the tariff term has been completed) unless the authorities believe that is safe to do so. (Many prisoners currently remain in jail well beyond the end of the tariff period set in court).
Chris