How old is he? I didn't move mine into an 18 x 18 x 36 until he was over 12 months old. He started in a 12 x 12 x 18 until he was 4 - 5 months and then I moved him up to an 18 x 18 x 24, before finally getting the large one he is in now. He loves his huge cage and uses pretty much all of it.
I had my jackson in a 16x16x30 cage until 10 months, and then I moved him to his luxurious 18x36x40, which is actually 2 large reptibreezes put together. He utilizes every inch of his mansion