New player joining my campaign and he wants to get an implanted commlink for his character. What I'm trying to figure out is does he need anything else to make it work? Does he need a datajack or sim module or both for it? I'd figure he wouldn't, because, well, this comm is wired up in his head already, but I'm not too sure.

Sim module is included in a headware comlink per the main book.
So, no, you shouldn't need anything else. But I like to have a datajack for beetles, knowsofts, mapsofts, etc.

Why is there an implanted version of the sim module if it is already included in the implanted commlink?

In case you want to stick with a datajack and the sim module alone without hooking up a wireless PAN.

Still, the stand alone sim mod implant is most likely a dead-end piece.

...Violet uses a datajack for connecting to her external Hot Sim module. This way she can pass through airport scanners etc. All they will pick up is her commlink implant (which is legal).