Grievous Bodily Harm is a very serious offence, the most serious level of assault in English Law. The sentencing guidelines suggest that substantial prison sentences are often required, particularly when weapons have been used to inflict the injury. However, this will depend on the circumstances of the case.
He should not agree to be interviewed without a solicitor present. He will qualify for free legal representation at the police station irrespective of his means.
If he is charged he should instruct a solicitor to represent him as soon as possible. Legal aid may be available, subject to means. If he is charged with a Section 18 Wounding/GBH then the case will be sent to the Crown Court for trial.