You should take him to the Doctor.
Because... it may just be a coincidence with the Benadryl.
Or, try giving him kids Claritin. Instead of Benadryl.
That's what I use. And it does not cause drowsiness.
Hives can be from anything. And just guesswork unless you pinpoint conclusively, what is causing it.
AND he is swelling.... this is NOT good. You need to take him to the Doctor.
My Daughter for example, is allergic to Kiwi fruit. It is HIGHLY dangerous in some people. And anything related to Kiwi, can also pose allergic reactions. Doctors know this.
Your Son's hives... seems pretty bad.
I would take him to the Doctor, AND since the Benadryl does not seem to help nor solve it.
I can't imagine taking Benadryl, since it causes drowsiness in some kids... and then him having to go to school, in that state of drowsiness.... for example.
all the best,
Susan