Son killed an unidentified bug on his arm. Today it's red, golf ball sized bite that itches and is "uncomfortable". Concerned or no?
Local reaction. This sounds like a local reaction to whatever type of insect this was. It could be a mosquito, fly, spider, or other type. If it's itchy, that is usually the sign of a reaction as opposed to an infection, especially if it occurred within a day. You could try giving him oral Benadryl (diphenhydramine) and if it doesn't improve or if the redness is worsening or he develops a fever, he should be seen by his doctor.