We are going to Oahu. But my son (9 year old) wants to visit the volcano park on the Big Island.....?

http://www.jackshawaii.com/eng/tours_5.html

so fly in for the day and either rent a car and go see it or you can book a tour like the above link

traveling to big island from oahu to see volcano with 9 year old? set aside at least two days, don't try 1 day trip. you don't mention how long entire trip is. i was on oahu for six weeks and left many thing undone. many things for 9 year old to see/do on oahu. he should be happy either way. (have you considered taking him next trip when he is older and can appreciate it more?)

You can land and drive out to Volcano National Park. You have to hike kinda long to get to the crater. Its awesome. When you come to O'ahu hike Leahi (diamond head) you can see Kona moku (district of kona, known as Honolulu) Ko'olau Moku (Kaneohe) its really fun.

that national park is nuts you can see lava and stuff and its like really cold and its like a rainforest

its a very unique place in the world