I'm guessing it's a cockerel, but I agree with everyone else that this one is a tough one! (Especially given that Sebrights are hen-feathered!) I'm mostly going by the comb size & redness, as compared with the other male and the females.
Being smaller than the other male: I've seen some chickens that are smaller or larger than others, even among the same breed from the same hatchery. So I would just assume he's a smaller one.
Since bantams are supposed to be small, does that mean the small one is actually a "better" bird?
I'm assuming the bird in question hasn't crowed, and hasn't laid any eggs.