Ok, with his lymph nodes swollen and the rash it does indeed sound as though he has an infection from the surgery. This needs to be checked by the oral surgeon that did the work.
The sedation or medication may also have well caused a related allergic reaction and this needs to be watched for signs of further deterioration. If symptoms begin to worsen or breathing becomes labored have him taken to the ER or Doctor asap to be checked. If tolerated he may also be given some benadrly to assist with the rash.
Stopping the Pred may also clear this up on it's own but has nothing to do with the lymph nodes swollen.
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