Light Brown Birthmark On Left Cheek, Only On Face, No Other Sunburns. Treatment?

My 8 year old son has what looks like a light brown birthmark on the left cheek of his face. It just appeared this summer. People have told me its a sunkiss and comes from the sun. I googled it and found nothing about a such sunkiss. He is perfectly healthy and has had no facial sunburns ever. I checked the rest of his body and found nothing else. I am just curious to what this might be and why it just appeared this summer. It looks like a light brown coffee stain.