to touch a sore spot, to rub salt in the wound
Falar sobre a doença do filho dele é meter o dedo na ferida.
Talking about his son's illness is touching a sore spot.
The image is biblical: doubting Thomas poking his finger into Jesus's wound (John 20:27).
Source: John 20:27.
Falando's Idiom Trainer teaches you to recognize idiom meanings and create sentences using them, so you can speak like a Brazilian.