I believe that fawn is synonymous with dun. But I've never personally worked with it. And I'm pretty sure it's dun with some other stuff. Not just straight dun. Usually.
If it was common, the hatcheries would be churning out garbage that carry the gene. Like Lavender Orpingtons. Which they effectively ruined in the US. If the two of you want to believe that bird is Khaki, be my guest. I don't see any point in wasting my time on multiple breeding projects...
So you noticed the color as a chick and had him in a pen to cull and decided on his "odd coloring" as an adult that you'd keep him to breed?
Recessive Chocolate (behaves like Lavender) is very common in the US in some breeds (see above). Dun (Platinum/Khaki) which behaves like the Andalusian...