Hi Sarah,
Welcome.
You still have a lot of work to do I am afraid. You need to try and trace where this man came from, ie, where and when he was born.
Then probably an even more difficult part, did he marry? If so he may have children by the marriage. If he did, it could make things a bit tricky as those children would be his next of kin.
To obtain Service records from WW11 you have to prove that a/ the person has died and b/ that you are the rightful next of kin.
So, as I say, things could still be tricky, but not impossible.
I have had a look on the CWGC site and there are no R Rains who were KIA in the RN up to 1946.
All I can say is good luck to you.
Salty