How To Treat A Big Bug Bite On Stomach?

My father had some kind of insect inside of his shirt yesterday and it began stinging or biting him repeatedly, a couple of times on his stomach and once on his hand as he removed his shirt. A couple of hours later there was a large welt on his stomach and just a small bump on his hand, so he took some benadryl. This afternoon his stomach is a bright red color and the welt seems to have grown, about the size of his palm, and it's warm to the touch.