Hey, Sept. in NY is a good time. Kids are heading back to school so family travel comes to an end and the weather shouldn't be too hot. I suggest if you are visiting any museums to get advance tickets, this way you beat the cue which can be very long at times. Also some museums are free or discounted on certain days/hours. Visit thier website for specific info. The museum of Science in Queens should be a cool visit for your son. Check out the magazine Time Out New York Kids Edition. It comes out weekly with loads of events that are kid oriented and sometimes even free! Hope you have a great time.