FictionalCharacter
What doesn’t make sense to me is why there’s any likelihood of a conviction. The other boy hit OP’s son too, in fact hit first, and OP’s son retaliated and the other boy fell, so it looks like only one punch by OP’s son. Despite this, and no suggestion of injury, OP’s son apologised to the other lad. Does this sound like the kind of incident that would lead to a charge for an assault or public order offence? And is that charge really likely to result in a conviction? If so I’m extremely surprised. I’m also surprised that the police aren’t interested in a signed apology from the other party who started this (incredibly minor) scuffle in the first place by throwing the first punch.
It could be that the other boy has already signed his apology or similar. And we are only getting one side of the story here - I'm not suggesting the OP is lying, but other people/witnesses may have a different version of what happened.
Also, if this 'fight' happened in a public place (a bar or similar) then it would be in the public interest to proceed with a public order offence. Society does not want fights breaking out in public areas.
If the OP's son does not want to go down the 'community resolution' route, then the next stage would be a PND ticket, summons or charge - depending on what they are eligible for.
It could be a case that there are other witnesses to this incident (staff maybe) or CCTV, police. So, if neither party want to pursue a complaint of assault, then a public order offence would be applicable with the evidence provided.