I believe old Mr. Radley was more worried about Arthur Jr. besmirching the family name than his son getting into heaven. Arthur's troubles began when he was arrested with some of the Cunningham boys: Mr. Radley must not have been pleased about his son's taste in friends, and when the other boys were sent off to the state industrial school to serve their time, Boo did not join them. Though
... it was no prison and it was no disgrace. Mr. Radford thought it was. (Chapter 1)
So, Boo was released to the custody of his father--a sentence of self-imposed exile inside the Radley house--"and Mr. Radley's boy was not seen again for fifteen years." When Boo next got into trouble for stabbing his father in the leg with a pair of scissors, Boo received a similar sentence. When it was discovered that Boo would have to share a jail cell with Negroes, "the sheriff hadn't the heart... so Boo was locked in the courthouse basement." His stay was only temporary, and Boo was soon behind the closed doors of the Radley house once again. Mr. Radley would soon die, but his intent--to keep Boo out of sight and try and maintain his family's once good name--was obvious. Even after his death, Mr. Radley saw that Boo's life would remain the same; Boo's brother, Nathan, would return to Maycomb from Florida to look after Boo.
Although old Mr. Radley was a "foot-washing Baptist," Calpurnia questioned the sincerity of his religious beliefs.
"There goes the meanest man ever God blew breath into...," and she spat meditatively into the yard. (Chapter 1)
Old Mr. Radley's place in heaven may not have been as secure as he imagined, but after Boo's heroic rescue of Jem and Scout on Halloween night, his place was certain.