(Guest)
The idiot who you are marrying changed his name recently ie after he attained majority ie after turning 18. Till that time, he continued to be Hindu as per law as his father is Hindu. Once he crossed 18, it is upon his wish to either remain in his father's religion/caste or follow something of his choice as per Law of land. So your kid now (future) will be Muslim as father is Muslim and may then can change his name/religion upon turning 18. But now if you change your kid's name to your dad's name, it will be illegal as per Law of land.
Think about compatibility issues, this is not intercaste marriage, it is interreligion/intercommunity marriage. The circus of name change, will be one of the most bothersome headaches for kids born out of such wedlock. I hope you are able to understand the gravity of the situation. Wishing you good anyways.