Why indeed. We could speculate all sorts of things from the OP's information and lack thereof.
Why has she written nothing whatsoever about what her son said when she talked to him?
We only know what she said and what his dad said to her.
Why don't we know what her son said?
As he's average academically he almost certainly has language...
Can he reflect?
Did he say " He was winding me up and deserved it, I'm not sorry."? That might be why the OP thinks he has no conscience, but actually is more likely to be defensive bravado.
It's multiple times more likely the year 5 child has an impulse control issue than a personality disorder, but if he did have a personality disorder it would need specialist intervention - you absolutely cannot remove a personality disorder by grounding the individual and taking away their toys/ tech! Nor can you address an impulse control problem with punishment successfully.
@Darksunshine talk to your child, but more importantly listen to him, then talk to the school - ideally class teacher and senco - about strategy.
He might need a behaviour plan including being aware of sanctions which will be implemented if he is aggressive again, but also needs to urgently include strategies for him to implement instead of hitting.
It's really questionable that you haven't told us what he said - even if it was that he refused to talk or was defensive.
Of course it isn't impossible he's being verbally bullied and lacks the social skills or confidence to deal with it in any way but physically.