Which is correct?
I am hungry, but not as hungry as him.
I am hungry, but not as hungry as he.
I am hungry, but not as hungry as he is.
Can you explain the correct answer to me. Thanks!
“I am hungry, but not as hungry as he is.” = “I am hungry, but not so hungry as he is.” is the best.
thanks! Do you know why? Do you know if the other options are also grammatically correct?
I agree with Haihao. No. 2 is probably also grammatically correct, but less natural. I’d say No. 1 is common in spoken and informal English but incorrect. “As” is a subordinating conjunction, which requires a subject (nominative) pronoun. “Because,” “while,” and “since” are also subordinating conjunctions. I was angry because he was hurt. While he was upset, I took it as unintentional. In your case, the verb “to be” is implied, and you certainly wouldn’t want to imply “I’m not as hungry as him (is).”
The same issue comes up with “than.” It should take nominative pronouns. “He’s much more generous than I” is correct. “He’s much more generous than me” is incorrect. “He’s much more generous than I am” is probably the most natural of the two formal options.
Make that “more natural” of the two options. 
Hi GM
The main difference between your three sentences is the level of formality.
I agree with Haihao’s assessment that “I am hungry, but not as hungry as he is.” is the best choice. It is the “best” because it is grammatically correct AND will sound “natural” in most contexts.
.
.
Your first sentence might be viewed as “correct usage” in informal contexts. However, many consider it to be grammatically incorrect.
Although your second sentence is grammatically correct, it will tend to sound quite stilted in all but the most formal contexts.
.
Thanks you all for your comments! I appreciate your assistance greatly! 