Point out the article and section that says he can't. An executive agreement is between our head of state and other heads of state. It's a political agreement rather than a legal one, and no president's successor is bound by it unless he or she wants to be. A treaty, on the other hand, binds all presidents unless the treaty ends.
There does not need to be an Article and Section in the Constitution which states the executive is NOT empowered to do a certain thing.
Our Constitution is structured in such a way that if a particular branch of our government is not empowered to do some certain thing, then that branch simply does not have the power to do that certain thing. It is not necessary for the Constitution to declare that it lacks that power.