They attack Solo because they don't know him. Once they get to know him, they will love him. Well, maybe not so much love, but they will accept him as a legitimate flock member.
Do this by first creating a safe enclosure in the run for him to spend days in. This will keep him from being injured while everyone is getting acquainted. If you had figured out a way to brood him in the coop or run instead of removing him to the house, he would already be a member of the flock.
The steps to achieve integration are:
1. A week or so of see but not touch to let everyone get acquainted.
2. Open up Solo size openings in his safe pen so he can come and go but none of the large chickens can get into his hide-out.
3. Keep his food and water in this safe pen so he doesn't get deprived of them while he and the big chickens all figure out his place in the pecking order.
4. Once you see he is accepted and has figured out how to interact with the other chickens, move him into the coop. You may need to referee for a few nights until he finds his flock approved roosting spot.
5. Last of all, you may need to help him learn how to go into the coop at night. But he should be able to pick up cues from the older chickens.
