I think you are exactly right. Those mean old hens!!! When he matures his status will change but it is a problem right now.
Another possible suggestion, Put up another roost separate from the hen's roost. Make it lower than the hen's roost but higher than the nest box so the hens are not as likely to move to it. Then set him on that roost after it is too dark for him to find his way back to the nest, using as little light and commotion as possible. I'd also suggest pretty short so that only one can be on it at one time, maybe 8" long. I have had older chickens leave their normal roosing spot with their buddies and move to another roost spot so they could pick on younger chickens I was integrating. Some of the younger ones started sleeping on top of the nest boxes instead of on the roost, just to get away from those mean hens. When he matures enough, he will move to the main roost on his own.
Good luck! It can be frustrating.