Please have your boy seen by a Pediatrician
for 4 days needs to be evaluated. A basic blood count will help.
Hope there are no other bleeding tendencies.
For fever you may give him Paracetamol
in correct dosage along with tepid sponging with warm water.
Till the blisters heal, do not use brushes- it will worsen the condition.
is likely to be due to the mouth lesions he has and due to improper cleaning or dehydration
that can happen in fever.
Do visit your Pediatrician, get your baby examined and proceed accordingly.
Make sure he drinks plenty of fluids- water, milk, fruit juices, vegetable soups, Porridge etc.
Hope my answer helps you.
Wish your son a speedy recovery.