It depends what actually killed the man, the punch or the hitting the head on the ground, and what the state of the victim's skull was e.g. 'eggshell' or normal. If the man had got up and lived, without serious consequences to his health, what offence would it be? ABH ? Had the prosecution got evidence that the defendant intended to cause grievous bodily harm, they would have charged this as murder. The defendant could not be said to have foreseen that death would occur. That the victim had Asperger's is irrelevant, unless there was evidence that he was chosen as a victim because of it, but don't expect the Daily Mail to miss a chance to use emotive detail in a story :)
I'd be surprised if the A-G applied to increase it; he might as a sop to those demanding it; or succeeded if there was an application. The man might have got 5 years, unappealable, but the A-G would need to be thinking of far more than that to succeed