Sudden cavity in 9 year old

[LegoTherapy]

Ds has a sore throat and I've just checked his tonsils and noticed a black area that looks like a hole on his first molar. He goes regularly to the dentist and has an appointment next month for a regular check up. No problems previously and despite me looking in his mouth to check his brushing regularly I've never noticed any decay. The tooth is t fully erupted yet.
Can sudden decay happen? He's really good with tooth brushing and we are both upset that he now has a cavity. Well, I'm pretty sure it's a cavity. Taking a photo is tricky because his breath steams up the lens 😬

A better photo

Bump

Yeah that looks like it needs a filling :(

I don't understand how it's developed so quickly. I feel pretty upset about it too, like I've failed him. I remember having several fillings as a child but they were in my baby teeth. I don't remember my parents even checking my brushing but my grandmother gave me lots of sweets. Thankfully my adult teeth are very good bar one troublesome tooth. Poor ds