I am going to 2nd Neslaw1's rec to do Six Gun City - 6 yo Boy, western themed amuzement park = No brainer! I even want to go back - LMOA!
Anyone of these places would be agreat outing for a kids up to about 8-10 depending on maturity levels guess. I know my girls would have out grown them by 9 or 10 but not all kids.
Once he gets a little older take him on the ZipLine at Wildcat mountain, or try to do the Gondola this year as part of the trip - Mt Washington Observatory and Tip Top House should entertain all in the party.