I’m going against the grain here - at 14 he’s an adolescent, not an adult and I don’t think it’s right to write him off as a violent abuser. Teenage boys bodies develop faster than their brain and that’s why it matters so much how a situation like this is handled. Who he is at 24 will be affected by how he was handled at 14. But it’s not a foregone conclusion.
Before making any decisions about birthdays I’d want to have a serious discussion with his dps, or perhaps just with your dc if that would be better. They’re all lucky you didn’t press charges, but you left it to them to handle and they haven’t brought it to a conclusion so I’d want to know what happened.
It’s not normal for a 14 year old to punch his granny. Calling him names and predicting lifelong criminality is beside the point. There’s something going on - it could be anything from violence in the home, poor parenting, to a psychotic streak. The last one being another way of saying he’s a little shit - and if he is, at 14, his dps should still be seeking out help and support.
You don’t know what happened after he got home. And you might feel differently if he was beaten up afterwards by a regularly violent parent, vs patted on the head and given extra time on his iPad. This is what I’d want to find out.
The dps are doing him a huge disservice by not getting him to apologise, and rebuild the relationship, regardless of what consequences there were for him otherwise. If he were my gs, I’d probably send him a birthday card, letting him know that I’m willing to talk about what happened, and that I love him. That’s not letting him off the hook or minimising the seriousness of it, but it shows him how to build a bridge.