With Nezha acting as a mediator, Red Boy can repay the debt in a relatively gentle manner. Nezha is bringing Red Boy and his mother to Flame Mountain for this reason.
Although the fire was caused by Sun Wukong flipping over the Furnace of Eight Trigrams, it originated from Doushuai Temple, and since Lord reported it in Lingshan, he took on this cause and effect. Now, Flame Mountain has been burned into a desert, with the surrounding climate altered by the Divine Fire, making it impossible for rain to fall. Human Sect disciples are scarce, and no one would come to clean up this mess, so Tie Shan bringing Red Boy is just right.
Secondly, Red Boy has been overly pampered by Old Li and Lady Yin at Lingshan, and this continuing indulgence is not a good thing. Plus, his playmate Jin Can Zi has grown up, yet he remains in childlike form, which is somewhat unsettling for him, especially since Jin Can Zi no longer plays with him.
