Please don't say any more about 'special medicines'. He needs to be able to come to terms with death (including his own!) as a natural part of the process of living.
It is quite a usual thing for children to start worrying about their own death at around this age, and I think the direct, honest approach is best:
Yes, we do all die. Most people live to a ripe old age and then gradually get poorly enough so that they die, but some people are unlucky and get poorlynesses at a younger age that are so bad that medicines can't save them from dying. Some people have accidents that are so bad that the doctors can't make them better and they die because of their injuries.
Then you need to do some reassuring (if appropriate and true!) you tell him that everyone around him is healthy and not poorly, that the adults are there to keep themselves and him safe so that accidents are not likely to occur and that if they do, the adults will know what to do.
Then tell him what does happen when people die...that their bodies are too poorly, or broken, or tired to go on living and so they die, and that is very sad for the people who love them, but that they are comforted by thoughts of what that person was like when they were alive, and by remembering special times with them...
Which is why I love our special times (you say to your DS), let's go and have one now...(then you eat lots of ice cream! 
).
He will come to terms with this, and then he will be ok about it. But you must be honest and truthful so he learns that when you say he can trust you, he knows that he really can.