'Self defence' can be very hard to use as a defence in court. Your boyfriend would need to show that he couldn't have simply run away (or locked himself inside the house). He'd then further need to show that he had no choice other than to use physical force. Finally, he'd need to show that he used the absolute minimum amount of force to remove the immediate threat to himself. (i.e. that he stopped hitting the other guy as soon as he had put himself in a position to being hit himself, e.g. by running away).
GBH with intent is a very serious offence. The maximum penalty is life imprisonment. The sentencing guidelines for the offence are on page 13 of this document:
http://www.sentencing-guidelines.gov.uk/docs/a ssault-against-the%20person.pdf
Those sentences apply to a first-time offender convicted after a trial. Judges can't go outside of those sentences other than in exceptional circumstances (which must be explained in open court), other than to knock off up to one third for an early guilty plea.
Based upon that document, your boyfriend is possibly facing a sentence of around
2 year 8 months imprisonment (of which half will actually be spent 'inside').
Chris