I think he's going to be a dark palomino. I would be shocked if that mane darkened up, so that leaves palomino and the double dilutes (cremello, perlino, smoky cream). His black skin and brown eyes eliminate those possibilities, so that's why I think palomino. I definitely don't think he'll be gray.
Here's a gray foal: Andalusian foal on Flickr - Photo Sharing!
He's a little farther along in the shedding process, but you can still see that his new coat is very obviously gray. I see dark gold on your foal's nose, eyes, and ears - not gray.
To answer your question, IF he grays out his mane will not get darker. He could only get lighter. The grays you see with dark manes and tails started out with them as foals.
BTW - Grays can be any color imaginable. Even pintos and Appys can gray. If you've seen more blacks and bays gray, that's just because those colors are common. The base coat doesn't have anything to do with the gray gene.