Cursing Islam is open disbelief, may Allah save us, and using profanity is one of the major sins. Your duty is to advise him and explain the ruling on cursing, inviting him to sincerely repent to Allah. You should be patient while doing so. May Allah guide him, as Allah (Glorified and Exalted be He) states:
And We have enjoined on man (to be dutiful and good) to his parents. His mother bore him in weakness and hardship upon weakness and hardship, and his weaning is in two years - give thanks to Me and to your parents. Unto Me is the final destination.
But if they (both) strive against you to make you join in worship with Me others that of which you have no knowledge, then obey them not; but behave with them in the world kindly, and follow the path of him who turns to Me in repentance and in obedience. Then to Me will be
(Part No. 26; Page No. 72) May Allah help you do good.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.