Hotone: -6C underquilt\4C topbag
Plusone: -1C underquilt\-6C topbag (+7C extra topbag)
As Keg and Bubba have mentioned, it's not unusual to get temperatures dipping below zero in mid October. My own advice would be to supplement your top cover and your son's bottom just to be sure. I've used a folded wool blanket right under my posterior to good effects during our winter hangs, and I'm sure a Thermarest would work just as well. A spare fleece throw makes for a good bit of extra covering on top.
Thick wool socks, long johns and a warm sleeping hat reserved only for sleeping are a must and Hot Pockets are a great idea as well. If you have one, a detachable down hood works really well too.
Finally, one of the keys to staying warm at night is good hydration. Make sure your son has plenty of beverages that he enjoys. The down side is, of course, the late night pit stops.