I think you answered your son really well and most importantly honestly. It sadly won't be the last time he learns about people in this world doing things that others/he doesn't agree with personally.
I think as a mother you have clearly done a fantastic job for your son to be able to approach and talk to you about things as it would have been far worse for him had he had to bottle all the unanswered questions up.
I'd keep an eye on him, carry on being there for him and maybe have a discreet word with school if your worried about his grief. If you do feel he is struggling maybe look into boks on grief or counselling Although you may have alreadyu tried these.
Take care and remember you are being a fantastic mum by the sound of it x
