We went to Kusadasi, Turkey this summer and it was absolutely amazing! One of the best places ive ever visited. If your really interested, PM me and i can send you pictures. It was over 100 degrees everyday though (this was May) so i imagine this time of year it would be quite nice... and not too hot but still warm. We stayed at a resort though and ventured out only a few times for the market and such... but the resort did have really good childrens entertainment and shows everynight.
ETA: Turkey was really really cheap to visit.. even in the summer.. we got an all inclusive package for 7 days from lastminute.com which included airfare, hotel, transport, meals and drinks (alcoholic as well) for around £300 per person... and the meals were gooooood. I imagine it would be even cheaper now
Also, I agree that Paris is a beautiful city and you could bring your stepson to Eurodisney as well.