I say dark bay. You can see the variation from body coat to the legs. Hard to tell from the photos but looks like he's got red in the otherwise black mane and tail. I've seen that on quite a few bays. Just like sometimes you see chestnuts with some black in their mane or tail.
I'd have to see his summer coat. We have a reg. brown gelding that turns more red in the winter. In the summer does he look black with brown points, or brown with black points? Look for reddening in the flank and on the nose. Bays have a black nose, and browns have a reddish/rust nose.
But from what I can see in the pictures, he has black nostrils... I'd call him a bay.
I would like to say dark bay because his legs look like they are black, but his mane and tail are throwing me off because they look brown, usually bays have black points with black mane and tails. Do you have a picture of him in the summer?