Soda did not know where Ponyboy was, and he was very worried about him.
After the brawl where Johnny kills Bob, Johnny and Ponyboy go on the run and hide out in an old abandoned church. Dally gives them directions, and when the coast is clear he comes and visits them. He tells Ponyboy that Soda is “takin' this mighty hard” and he didn’t believe Dally when he told him he did not know where the boys were.
I wish you'd come back and turn your selves in but I guess you can't since Johnny might get hurt. You sure are famous. You got a paragraph in the newspaper even. (ch 5, p. 82-3)
In the letter, Soda says Darry is sorry for hitting him and says they were worried about him. Soda is very worried about his brother, and feels frustrated that he can’t help.
In Chapter 5 of The Outsiders, while Ponyboy is hiding in the abandoned church, Dally brings him a letter from Sodapop. The letter says that Sodapop and Darry (Ponyboy's brothers) were very worried about Ponyboy when he and Johnny were missing after a dead boy was found in the park. In the letter, Sodapop says that Darry is sorry for hitting Ponyboy. He also says that he can't believe Johnny could kill someone and that the police came by to question Sodapop and Darry, but they don't know where Ponyboy is hiding. Dally won't tell them where Ponyboy is hiding, which is really upsetting Darry. Sodapop says that he wishes Ponyboy and Johnny would turn themselves in to the police, and he also tells them that their story has been in the newspaper. From the letter, the reader knows that Darry truly cares for Ponyboy, even if he doesn't always act like he does.