Just a few points.
1) If what you are saying is true then there is not a court in the land that will convict your boyfriend. All cases at court have to be proved beyond all reasonable doubt. If he has medical evidence to say he is injured (probably the police surgeon, if he was arrested) then this will go in his favour.
He will have to prove self defence with what is known as "the pre-emptive strike". He will have to convince the magistrate or the jury (always opt for crown court!!!!) that he acted reasonable and was in fear of an imminent attack.
Regarding the counter alegations, it will be best to wait after the court case. He should lodge a formal complaint with the inspctor of the officer in the case. Any inspector of that force will suffice however.
Further, and this is not a racist slur, was the taxi driver Asian? If he was then make sure you boyfriend feels it was racially motivated. I can guarantee you that if this is lodged you will get very prompt response.
For the crime to be lodged as racially motivated ANYBODY can feel it was. Whether it was or not, it does not matter.
Give the Asians a taste of their own medicine and keep stressing the taxi driver was racist, racist, racist. If he was white, then forget my last.
As for taxi drivers, like ALL professions there are good and bad.
Also although your boyfriends solicitor would have based his defence on it, your boyfried should learn Sect 3 of The Criminal Law Act and although do not repeat it in court, should word his answers ALWAYS relating to it.
http://en.wikipedia.org/wiki/Criminal_Law_Act_ 1967
Scroll down to sect 3.
Good luck.