I often find that if I start the conversation with my own worries about the cancer (in your case your step mum’s)
It opens up the topic but concentrates it on your worries rather than your step brother’s.
It is so easy for it to become “the elephant in the room” with everyone skirting around the subject.
This should help him to speak about his own concerns.
He is obviously aware that you know about his mum's condition so perhaps you could start the conversation and ask him if there is anything practical that you could help him with, may be that will give him the opening he needs to talk about it
It is lovely that you are there and ready to support him