On the Saturday morning of Labor Day weekend in 2014, an 82-year-old man named Frank Stack hobbled out of his house in Elmhurst, Ill., a quiet suburb of Chicago, and drove 15 minutes northwest to the group home where his 48-year-old son, Francis, lived with five other men with developmental disabilities. David Clark, the supervisor on duty, knew the whole family well and was friendly with Stack. ‘‘You’re here on Saturday,’’ Clark said, surprised. Stack normally took his son home for dinner on Sundays. ‘‘Mom wants to see Slugger,’’ Stack said. It was a holiday weekend, so a change in plans was no big deal. Stack promised to have his son back by 5.