So sorry you are going through this. I think all you can do it sit down and chat together. Be matter of fact, answer his questions and be honest. Talking in the car is good as he can ask questions without having to look at you. I have bought some nibbles and driven to somewhere pretty before for 'big' conversations. It means there are no distractions and feels like a safe place to talk.
As he is already worrying he will probably be very glad to have things out in the open and to know he can talk to you about any concerns or worries, often what you imagine is so much worse than the reality.
My eldest is 8 and rarely asks questions. She does devour appropriate books though, Macmillan may be able to point you in the right direction for specific books for your illness and his age range, your local hospice may be able to offer support too.
Also talk with the school. They may allocate an LSA to have a special bond with him, my daughter used to have an hours session once a week with some other students in the school who needed emotional support, cooking, board games, art activities, general easy nice activities for relaxation and to help build relationships so the children felt confident that there was someone they could talk to.
My youngest asks questions regularly, sometimes ones that are very hard to answer, usually at bed time when she is lying in the dark.
Wishing you both all the very best. A big hug to you both.