Here is an explanation of use of force legislation in the UK: https://www.reddit.com/r/policeuk/wiki/mythbusters#wiki_use_of_force
Section 3 Criminal Law Act 1967: Use of force in making arrest, etc.
A person may use such force as is reasonable in the circumstances in the prevention of crime, or in effecting or assisting in the lawful arrest of offenders or suspected offenders or of persons unlawfully at large.
This is an any person power which allows people to use force to prevent crime and to assist in arrests.
Common Law
"A defendant is entitled to use reasonable force to protect himself, others for whom he is responsible and his property. ... It must be reasonable." - (Beckford v The Queen [1988] AC 130)
A person may use such force as is [objectively] reasonable in the circumstances as he [subjectively] believes them to be
"A man about to be attacked does not have to wait for his assailant to strike the first blow or fire the first shot; circumstances may justify a pre-emptive strike."
This is a common law, so most of the wording comes from case law surrounding the subject. The power grants any individual the power of self defence, as long as the force used is Reasonable, which means the use of force has to be proportionate to the threat.
To use force, especially preemptive strikes, one must have an “honestly held belief” that that the assailant is going to subject them to violence.
The main focus of police use of force is that it should be Proportionate, Lawful, Accountable, Necessary and Ethical. This is a mnemonic called PLANE. Any force police use has to be justified by the officer; the use of force will be expected to fall within these categories.
Essentially you can use any force you want in this country, so long as it was "reasonable" and "proportionate". In this case, if could argue that his force was necessary by justifying it. Impact factors like his age, his stature, him feeling scared, how big the attackers were and various other things will be taken into account.
He has been arrested as a means of investigation, so the police can take forensic samples and interview him under caution. The police aren't concerned with finding him guilty, all they want is to gather as much evidence as possible for the CPS, so they can make a charging decision. If all of this is above board, he may not be charged by the CPS and even if he is I can't see this going very far in court. Of course we don't no all the facts of the case currently, but by the sounds of it the force used was proportionate.