Usually during teething
,swelling and sores in mouth are common.
Gargle frequently with warm saline.
Clean your child's gumpads after every feed using moist cotton.
Maintain oral hygiene
I would suggest you to give varieties of food stuffs to your child.
You can give cooked carrots,potatoes etc.
Vitamin b-complex syrup can be given after consulting with a dentist/physician.
If symptoms persists,please do visit a dentist.