If it is less than a chickpea, it does not invalidate his fast because it is a small amount that cannot be avoided, so it is considered like saliva. However, if it is the size of a chickpea, it invalidates his fast because its presence between the teeth is unusual and can be avoided, as mentioned in Radd al-Muhtar 2: 98, and Bada'i al-Sana'i 2: 90.