Yes, Langshans are definitely slow to mature. Even though he gets along with your other chickens, he might still be a bit of an outcast since he has none of his broodermates to hang out with. And older hens will often keep a young cockerel in his place, so it might take several more months until he develops the maturity and confidence to act the role of flock rooster. But I would think that without any other roosters to compete with, he will get there eventually.