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.