I would say yes. Its ok to accept that cash because he is your relative plus he is helping you without expecting any Haram in return. So he is not buying you or buying anything of you. He is just helping you.
Its like finding some money on the street or somebody you don't know give you some money to help you. I don't see any problem accepting that money. But make sure, you also give some of that money to poor people to show your good intentions off of that money.
If you don't accept that money, then you should not accept any money from anybody other than Muslims. However many scholars even prophets made trades and deals with the people from other religions. Don't forget that for a Muslim, we all believe in the same God. Only prophets are different.
Plus i think is he a good person or at least a person with good intentions because there are not many millionaires out there giving mass amount of cash to their relatives just to help out.
Ps: If you don't want the money, you can tell him to send it to me :) JK