So what is exactly your schedule of feeding? You mentioned you leave the kibble out, is that correct? Would your pup eat raw twice a day on top of that? And now he only eats it at night? Or is he only eating the kibble? If his appetite is slowing down he could only be going for the food that has a bit more flavor to it (kibble) as raw food can be very bland in comparison.
My dog, Isla, is about 6 lbs and only eats once a day minus snacks (bits of whatever meat we may be eating that's leftover). I don't feed her any kibble, only raw, and she will only eat once a day. If she's in the backyard with the big dogs then she will eat most anything I put in front of her just so the big dogs don't get it!
But when we went to the lake this summer we didn't bring the big dogs and Isla didn't eat her raw for 2 days being there wasn't any comptetion! I was a bit stressed! She did get scraps from our plates (only what's dog appropriate, mind you) and she eventually ate on the 3rd day but I had to sit next to her. She also didn't eat as much in quantity the entire trip.
So maybe yours doesn't need as much food as you think. Has anything else changed? Energy? Sleepiness? Mood? If not, don't worry too much. Maybe he's just leveling off. If you're determined that he eat raw, maybe you'll have to cut out the kibble so that he'll eat the raw. Good luck!