he looks a little big to be a serama?
unless he is one of the larger classes
did you weigh him?
or put him next to another bird that you know the breed for comparisons?
Hi! Is he upright most of the time? He should have his chest stuck out and his head back a bit and his wings pointing more or less straight down. So, hard to say from the pictures... A good Serama cock will have the back of his head nearly touching their tail. While they are standing around. Your guy is looking around and at the ground so I can't tell....
Here are a few of my young cockerels for comparison. They will fill out more in the chests as they get older.