What happens with Billy/Greg’s dad in the summer of 1956?

Greg's father, James Anthony Williams, spends the summer of 1956 at the Logansport State Hospital in an addiction rehabilitation program for alcoholics.
Greg and his brother, Mike, are initially sad that they won't see much of their father during the summer. However, they are encouraged when he succeeds in completing the program at Logansport by midsummer. After Logansport, James is transferred to the Nash Rehabilitation Center in New Castle. There, he is hired as a clerk at the center's business office. The center also provides James a small trailer to live in.
Eventually, Mike and Greg begin visiting their father at Nash. The boys spend every possible weekend at the rehabilitation center. It is during this time that James advises Greg to do everything he can to pursue his law ambitions. Noting his father's intense efforts at achieving sobriety, Greg begins to hope that life will change for the better for all of them.
Essentially, Greg's father spends the summer of 1956 trying to overcome his alcohol addiction. During this time, James endeavors to reconnect with both Greg and Mike.