I have learned that around 7th century BCE there was a king who basically said- "We're all gonna be monotheists now."

Perhaps you are thinking of the Egyptian Pharaoh Amenhotep IV, also known as Akhenaten, but he lived during the 14th century BC, not the 7th century.

He originated a form of monotheism known as Atenism, the worship of a Sun god. It didn't last much beyond his own reign, so it didn't "stick".

If you mean some other king in the 7th century BC, I don't know who that might be. Could you look up the name?

Though it may seem counter-intuitive, there is actually a spectrum of possible forms of worship from non-hierarchical polytheism on the one end (there are many gods, all of equal standing) to complete monotheism on the other (there is only one god, other ideas of gods are false). Hebrew monotheism originates from Canaanite polytheism; an intermediate phase was something like monolatrism - the belief in several gods but the particular worship of only one. To use an example you may be more familiar with, consider the role of Athena for the city of Athens - she was the patron deity of the city (the names are related), and the Parthenon was dedicated to her. However, that is not to say that Athenians denied the existence of the rest of the Greek pantheon by any means, merely that Athena held a special place in the Athenian heart. Early Hebrew monotheism likely took a similar form - Hebrews acknowledged the existence of other gods, but merely held YHWH (related to the Canaanite god El, from which words like "Elohim" and "Allah" are related) in a slightly higher regard as patron deity of the Hebrew people. Over time this evolved into henotheism (worship of only one god, but not complete denial of other gods), which makes an easy transition to monotheism.

Source:

Caquot, André. "At the Origins of the Bible". Near Eastern Archaeology, Vol. 63, No. 4, The Mysteries of Ugarit: History, Daily Life, Cult (Dec., 2000), pp. 224-227