Your son does indeed have a lot on his plate, and so do you. Good luck to you. Certainly you are right that, in today’s world, the issue you raise occurs in many households. There are several good books out now on step-parenting. Check one out from your local library, or browse through your favorite bookstore and pick out one that seems to fit your situation most closely.
As far as preparing your 7-year-old son (who, I gather, lives with his mother and not with you and your new wife) for a new step-sibling, I wouldn’t do anything until your wife is actually pregnant. Oh, you might remark casually to him some time when you are together, “You know you’re your step mom and I hope to have children once I finish with these chemo treatments. How would you feel about that?” He will probably say something like, “I don’t care.” Even so, you will need to reassure him that a new baby won’t make you love him any less, that he will always be your first son, etc.
Once your wife is actually pregnant, let him participate in the progress of the pregnancy just as he would if the expected baby were to be his full brother or sister—comment on size changes, allow him to feel the baby’s movements by putting his hand on the mother’s abdomen, discuss possible names with him, etc. The fact that you are already thinking about this makes me think that you will do a good job of preparing your son.
Finally, may I put in a plea that you not abandon financial support (as well as emotional support) of him when a new baby comes. Alas, that is also a common scenario in today’s world.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.