Hello Mr Amit ,
Welcome to HealthcareMagic forum.
Preterm babies have a risk of neurological problems like delayed development and 
seizure disorder in the future.
Meningitis in preterm infants can additionaly affect the brain development  and cause convulsions or 
paralysis in the infant.
As you mention that your baby had a normal EEG and  
MRI  after the treatment then chances of significant 
brain damage is unlikely . However  there can be minor disturbances in the brain function causing convulsions  in future. Such convulsions are less likely to occur after 5 years of age but since now at 3.5 months your child has had one episode you need to be alert and get it thoroughly treated.
The next best step is to consult a pediatric 
neurologist or 
epilepsy specialist who will carry out detailed neurological assessment of the child , repeat some tests like EEG and MRI and start an appropriate anti epileptic medicine if required. This will decrease the further risk of seizures and regular follow up will ensure better neurological  prognosis.Please do not worry , with timely and proper treatment the child can have a near normal future ahead.
Hope this answers your query.
Wishing your baby good health .
Regards