Thank you for your query.
The swelling of the lip is very unlikely to be an allergic reaction
. Most of the time the lip can swell following an insect bite. The swelling should subside over the next day or two.
You can even look inside the mouth for any evidence of injury
like cheek bite which can give rise to painful swelling. However there is no mention of pain so it is most unlikely.
I hope I have answered your query to your satisfaction.
Wishing your son all the best.