Kids this age can understand much more that we think. In fact they sometimes understand and have a much healthier reaction than adults.
Your story reminds me of my son at that age.
When he was 5, his grandfather had a second heart attack, which we knew he will probably not recover from.
He asked many questions, so I decided to explain it to him in a very practical way. I explained that there are vessels that bring blood to the heart, and his are clogged. Just like pipes of water can be clogged. I reminded him of the time our water pipes were clogged and the plumber came to fix them. I then explained that if there is not enough blood to the heart it can't work properly. He was quite content with the explanation.
A few days later we had a dinner party at the hospital I worked in and we happened to sit next to the head of the CCU (a very strict old fashioned physician who would hardly greet you in the corridor). My son started a conversation asking him what he does, and he told him that he was a cardiologist and takes care of people with heart disease. "That's great", my son said," then you can take care of my grandfather. His heart vessels are clogged and you can probably open them, just like the plumber did with our pipes". Needless to say that from then on the head of the CCU would greet me every time he saw me, and say-what an amazing son you have.