This condition is called early childhood caries means there is cavities in multiple tooth which will progress if not treated on time and further progression will lead to pain in the tooth ,preserving the deciduous tooth (milk tooth) is really important for proper alignment of permanent tooth .
If there is pain currently medication can be given
Rx
Ibugesic plus 1 tsp 2 times a day for 3 days
Next Steps
Visit your dentist for cavity filling of the tooth and a radiograph is required to see the progression of cavity if it is really deep then treatment plan will change (pulpectomy) followed by cap placement (lower jaw tooth)
Health Tips
Maintain oral hygiene (brush twice )
Avoid sticky food
Rinse his mouth everytime after eating