what punctuation error is in this sentence, As i saw him prepare, i whispered "he's is going to blow it"

If you were to put an apostrophe 's' in the word he, then the "is" isn't necessary to input. The word is is already counted in the word "he's". Unless you remove the word "he's" and leave it as, "Is he going to blow it". Indicating that you are indeed asking a question. On the other hand, I think the apostrophe is the punctuation error.

He's is wrong it should be only "he"