His comb and wattles look like they're full grown and deep red like he's somewhere between 6-9 months. His tail feathers still have some growing to do so he's not full grown. Hard to tell. Is he small like a bantam? Bantam cocks will weight about 900g when mature. A standard Cochin will weigh close to 5K. That's probably the easiest way to figure out how old he is (as long as he isn't a runt)