I'd get a used 1100 or 1187 for your son, but that's just me. Still have mine from when I was his age and still enjoy shooting it. Wing shooting with a pump and staying on target on follow up shots, especially for new shooters, is hardly a small task and can get frustrating pretty quick.
On your last question, it depends on what chokes are included with that 870 from Academy. It's not clear (to me) on their website.
You'll want and have quick need for the three most common, I/C, modified and full.
I'll look into the 1100 and the 1187. Was yours a 12ga or 20ga? How much do the extra chokes cost?