Take him to the Ancient City in Samut Prakan.It is a huge place where they have built copies of temples, ruins and other important buildings from all over the country.
And when you are there, it is not far away from the crocodile farm.
Only about 30km from Bangkok, so it isnt so far away.
Ayutthaya i think is about 150-200km north of Bkk. Takes about 2 hours to go there,... Never been there, but people say it is beutiful. I know there are busses that goes there, just ask at the hotel...Or rent a cab. Shouldnt cost to much..
http://chiengfa.com/thailand/bangkok/ancientcity.html
http://www.hotelthailand.com/tour/bangkok/tour4/
Found 2 links...