Here's why:
* "Assuming" is a gerund, which functions as a noun. It can't be the direct object of the verb "appreciate".
* The sentence needs a dependent clause to explain what you appreciate about his assumption.
Here are some ways to correct the sentence:
Option 1 (Simple and direct):
* I appreciate him for assuming he would be leader.
Option 2 (More nuanced):
* I appreciate his assumption that he would be leader.
Option 3 (Adding context):
* I appreciate him taking the initiative and assuming he would be leader.
Option 4 (Using a different verb):
* I respect him for assuming he would be leader.