The boy's mother may have been an ebola survivor herself.
The 42 days number came, if I recall correctly, from a survey of patients who had been exposed, and represented the largest time known between exposure and showing symptoms. I think it should be revisited. And we know now of reservoirs of live virus in survivors lasting months after they are nominally cured, in semen and the fluid of the eye and who knows where else. If it got into those spots, it can get back out again. How long are antibodies good for? Can a milder infection produce "Typhoid Marys?"
"Ebola-free" is too strong a claim. Unfortunately.