Hello
This can be common cold, Viral fever or something else. We will need more history to come to a final diagnosis. As he already has history of febrile convulsions, the best option is to immediately consult a pediatrician whenever he gets a fever which doesn't resolve with single dose of PCM. PCM mostly provides symptomatic relief within 30 mins to 6 hours, if you don't see an improvement, please consult a doctor .
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Next Steps
consult a doctor/pediatrician