How old?
I'm surprised he isn't crowing yet. Must be close!
The giveaway signs are those long pointy neck feathers that have begun growing, the longer ones starting to grow at the sides just before the tail and of course those lovely new tail feathers starting to grow in. Topped off of course by that distinctive comb and his impressive wattles.
It's definitely a gorgeous boy, if you have any more chickens that you are unsure on the gender of, why not look at or post a few pics on the big fat is this a rooster? Thread
https://www.backyardchickens.com/t/944086/the-big-fat-is-this-a-rooster-thread