Answer: You are in the area between hardiness zones 9 and 10. Unfortunately, cold fronts don't take the time to read zone maps. Each year is different and there are many micro climates throughout the country and even around a city where buildings, bodies of water, the direction a slope faces, and many other factors will affect temperatures.
Consult your local County Extension Office and knowledgeable nurseries for information on the many plants which will be best for you...and then get ready to run outside with blankets some future winter to protect a prize, but marginally hardy, plant from that abnormal cold spell!
Regarding the apple tree for your son. Many apples will grow well in his location in Illinois. Among the best fresh eating varieties are Gala, Braeburn, Mutsu (also called Crispin), and Jonagold. There are many other wonderful varieties to choose from. You might also choose a disease-resistant variety such as Liberty.
Two apple varieties are usually required for cross pollination. Of those I mentioned above, Mutsu is pollen-sterile and will not pollinate another variety (this would require a third variety to be planted for all to have fruit).
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