The iguana is puzzled when the boy is not afraid of him. In fact the boy seems amazed by the size of his pet and rushes over to pet him. Since iguanas are not big meat eaters the boy is relatively safe. Then the iguana notices the assortment of vegetables in the boys hand. The boy can see were his eyes are focused and feed the lizard. It was a delicious meal, much better than what the pet store feed him.
The iguana was grateful to the boy. Since the child liked him at this new size, he figured the boy would like it even more if he was larger. He would have to get out of the room, it just would not do to crush the boy before he could be properly impressed. So he headed out the room and continued out the house with the boy following behind him.
When he was outside the iguana continued his growth. The iguana can see he was correct about the boy. The boy smiled and ran up to hug his thickening neck. At four times the height of the boy the iguana ceased his growth. The iguana had never received this amount of attention from anyone and found he enjoyed it.
He made up his mind to keep his boy happy, but how to do that. He could continue growing himself. There was also a pet cat and dog in the yard. The boy might be happy to have a collection of giant pets.