I would pursue a relationship with him... as you said... the cancer and the impotency are curable, will only take a few months. This is at the beginning of the relationship where it's more favorable to go slowly with physical intimacy anyway.
Just don't become overly sympathetic towards his medical condition to the point of changing who you essentially are.
Wish you the best of luck...
"The weakest soul, knowing its own weakness, and believing this truth that strength can only be developed by effort and practice, will, thus believing, at once begin to exert itself, and, adding effort to effort, patience to patience, and strength to strength, will never cease to develop, and will at last grow divinely strong."
- James Allen