In modern English usage, Caelan for a boy probably represents a name coined for its sound-- in the late 20th and early 21st century, 2-syllable names with a long a sound and an -n ending are all the rage (i.e., Nathan, Jaylen, all the Jayden/Braydon/Caden names). Compare Kaylin for a girl. However, if you really want a meaning, Caelan could stand for an anglicization of Caolán, an Irish name derived from the word caol, meaning "slender."
See Also: Cailan, Calen, Kaylin, Cailin, Cailyn, Caelyn, Cailynn, Calen, Caillan, Kaelin (boy), Kaelin, Kaylyn, Kaylynn