It's in Chapter 5, when Razix starts to flirt with you on the bridge and Kol interrupts him, asking for coffe. Then you two go to the galley and you get the option of playing coy or squeeze Kol's arm.
Literally do either and he will get you in a headlock, bending his head down to your nexk, showing his teeth. That's the CG you'll get to see, and that's "Initiating a mating ritual" in his culture.
You don't really need to make any special choices, I think, during Kol's route to get this CG.
But if you have a missing CG for Kol, maybe it's the one, where you're going down on him in the shower? You get that one after getting stabbed, then saved by Doc, then taking a shower in one of the Med Bay showers, when Kol asks you if you have room for one more. Here, just say "Yes, please!" and you'll get that one.
Also, you probably won't need this, because you posted this comment a while back XD But just in case it helps!