Here's why:
* Subject-verb agreement: The sentence has two subjects, "this guy" and "John," but only one verb, "thinks." The verb should agree with the closest subject, which is "John." Therefore, it should be "thinks."
* Missing connector: The sentence needs a conjunction to connect the two subjects. It should be "This guy and John thinks he is..."
Here are some grammatically correct ways to rephrase the sentence:
* This guy and John think he is...
* This guy thinks he is... and John thinks he is...
* Both this guy and John think he is...