Thanks for asking the query,
Fever along with blisters needs to be evaluated, a basic blood count will help.
For fever you can give paracetomol in correct dosages, please take your baby to the Pediatrician
Do not brush the teeth till the blister heals as it may worsen the condition.
Take your child to the Pedodontist and get the thorough clinical evaluation done.
along with pus formation can be due to improper oral hygiene
, scurvy, bacterial and fungal infections. It can be a serious sepsis
If your child is taking some antibiotics for fever it can be due to reaction to antibiotics and other drugs leading to ulcers on lips, gums and tongue known as steven johnson syndrome
Make sure that your child drinks plenty of drinks and water, milk, fruit juices.
Hope this helps out.
Wishing your child a healthy recovery!