*I don't know how you deal with this. But do have experience of children lacking in empathy.
Has he never shown any empathy at all, ever, or could it be a phase?
My children do sometime seem to delight in winding each other up, and sometimes turn a blind eye if one of them gets hurt, but this sounds different, tbh.
Sometimes, my eldest daughter shocks me with her lack of empathy, but I think I'm the opposite, and have too much (!), so I think it is even more shocking to me that it would be to others, iyswim.
It's not that she is completely devoid of empathy,though, just that she was last in the queue when it was being dished out! 
How is your DS when he hurts himself, feels upset? Does he come to you to comfort him, or for a cuddle? Or is he completely self sufficient? Is he affectionate to you?
Do you have pets? If you do, I think the way they treat pets is quite telling. Though mine can be pretty mean to each other at times, my youngest three adore our dog, and have never, ever been mean to her either verbally or physically. They are very gentle, kind and caring
towards her, and wildlife in general.
My eldest DD can take her or leave her, tbh.
Youngest DD is currently going through a phase of loving winding up her sister. I can see it in her eyes that she loves it. She will also hit her and then when questioned will say things like "But she was annoying me...I didn't hit her that hard! 
.
Sorry for rambling!