I got married recently and wanted to make myself something small and skimpy for the big night. After searching the net high and low and trying out a few patterns (which looked like they were designed for porn stars gone to seed), I finally got fed up and decided to make my own, and this is the result!
This was done on US 6 needles and knit up fairly quickly. It comes with the advantage of fully detachable straps about the waist and betwixt the legs. More importantly, it will fit, unlike every other pattern I have found for free on the net.
The pattern is easily modifiable to fit guys of all sizes (both waist and crotch). My one piece of advice, particularly if your guy has never worn a g-string before: it can chafe in the back, so you may wish to use softer yarn (i.e. not man-made) and make the linkage in back a bit loose.

I’ve also posted a version sized for 00 needles, for those insane enough to want to do things on that size needle (like me!).

I hope you find this pattern helpful and fun (in more ways than one >.>). Enjoy!