Where in the U.S. Constitution does it guarentee a foreign leader the right to free speech?
It prohibits Congress from passing laws abridging freedom of speech. Doesn't say anything about whose freedom of speech, unlike the later prohibition involving "the right of the people peaceably to assemble, and to petition the Government for a redress of grievances." or ".. the right of the people to keep and bear arms" or "the right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures".
Since he's not part of the group "the people", we should search his airplane. Might be really interesting what we'd find there. But as you can see, he also has no right to assemble, which is what he did today at Columbia. He can talk all he wants, but he has no right to *be here* at all.