S1500

Maybe a trip to archive .org and playing some emulated classic games would spark some inspiration. Also, if you know C you can make actual Coleco, Genesis, 2600 and even SNES games (among others).

And if he knows C, it isn't a huge step down to BASIC which enables him to program for yet more classic systems. I think the audience relative to effort is greater if you go for a real retro system than a modern system with a retro feel.

C# is in the C family of programming languages, but C# and C are very different. C# targets the CLI, so you're not going to get C# programs to run on vintage platforms. Of course, retro-style games on modern platforms is certainly possible.