You can give paracetamol to reduce his pain. Whether he should be given metrogyl or not or some other antibiotic instead will depend upon the condition of the tooth. Kindly do not self medicate.
Next Steps
Since you already have done his treatment before, share the xray if you have any. Also share the tooth images on personal chat for more precise suggestions.
Health Tips
Let him brush twice & rinse well after every meal.
Avoid sticky, sugary cavity promoting foodstuffs.