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It sounds like he might be having an allergic reaction to something. It could be pollen, a food, an insect....
Many vet recommend giving 25 mg of benadryl (12.5 mg for a small dog) and see if it helps.
If you don't see some improvement soon, you will want to give your vet a call.
Keep an eye on his breathing and be sure that air is moving freely. Some dogs have some respiratory distress when having an allergic reaction.
Let me know if you still have questions.
I agree with your diagnosis, if it's any comfort.
Keep in mind that they can have allergies to insects and this is a pretty common culprit when you see this kind of swelling.
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