Son killed an unidentified bug on his arm. Today it's red, golf ball sized bite that itches and is "uncomfortable". Concerned or no?

Large local reaction. This likely not a big problem. Use ice, diphenhydramine, and topical hydrocortisone to treat the swelling and itching. If your son's symptoms spread to other parts of his body, or he complains of difficulty breathing take him in to be seen by a doctor in person.
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.