Hi Sister! Your husband has a lot of scary things going on, and pain can make a person miserable, even if they believe in the Lord with all their heart.
I understand how hard this must be for you: you love him, you worry about him, and you want him to hold onto God in these moments and he doesn't seem to be doing that. Put your own life vest on first, as they say. Study God's Word and become SO in love with Him that nothing can shake your faith. Reach out to other women to pray for you and comfort you, because though you are not the one with cancer, you are struggling too.
Also... allow your husband his feelings, as you pray that the Lord reach every part of him on this journey to regain his good health.