Are you sure it's the entire key he sent you, has he tried to redownload the private key? Can you get him to download the entire wallet data and try to pull the private key from there. (Only if no one else knows how to do the checksum). You might want to contact blockchain they can get the encrypted wallet file to you in its entirety.

Have you tried inputting it into a wallet so see if that'll add the checksum?

We could always try to dig through an old electrum source code to see how the checksum is generated/checked.