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It is not unusual for children to keep temps under 100 for several days with a simple cold. The fluid you hear in his cough is likely related to drainage down the back of his throat, which often gets worse for the first 3 or 4 days of a cold.
If he developed ear pain, chest pain or headache with his cough and congestion, it would be worth getting him re-checked to be sure he hasn't developed an ear infection, sinusitis or bronchits/pneumonia. Usually these things would also cause the temp to rise (over 101), but not all children have fever with these complications.
If his temp stays fairly low as it is now and he has no pain or difficulty breathing, it is likely just a cold that needs to run its course.