Thanks for the question.
Only Christ can heal your nephew, everyone else is simply an intercessor. You can pray a novena to St. Joseph, as this is the year of St. Joseph, and if God wills it, he can be healed. The pastor at my parish was healed from terminal cancer about two years ago, a real miracle—doctors had expected him die in six months, but he is cancer free today and has been doing fine. The whole parish was praying for him. God still heals, so don't give up. I would also have some Masses said for his healing. You can also do that through your parish for a small stipend.
I know what Jesus said about the mustard seed analogy, but miracles are not automatic. Somehow these sicknesses we go through are part of the plan. They work for good for those who love God, and we have to trust God knows what He is doing no matter how our prayers are answered.
When the little seers at Fatima brought requests for healing to Our Lady, she said,
"Some will be healed, but others, not."
God sometimes answers no, and it is not out of cruelty or indifference, but actually out of love. Your love for your nephew will lead you to holiness in this difficult time if you let God lead.
Lastly, we will pray for him too. I know it is possible for God, so let's storm Heaven and trust in Him.
[Apostolic Letter on St. Joseph by Pope Francis]