Hi, thanks for using healthcare magic
It is likely as you stated, that the swelling is related to an allergy but if it was related to medication that he was using repeatedly he would be experiencing persistent allergic symptoms.
The allergic symptoms would not be intermittent since he has to use the insulin
4 times a day and would be constantly be exposed to the stimulus.
To determine what specific allergen is provoking this reaction, you may want to consider taking him to have allergy tests
done. These can be in the form of a skin test or blood test
It would allow you to eliminate this allergen, if possible from his diet or environment.
I hope this helps