What's the rhyme scheme of "Lord Randal"?

The rhyme scheme is actually AABB, but it's an issue that's open to interpretation.

If you speak American English, then when you read "Lord Randal," a Scottish ballad poem, it will seem to you to have no rhyme scheme at all. Every stanza has four lines, and those lines end in the same words every time: "son," "man," "soon," and "down." Read those aloud and hear how they don't exactly rhyme, although there's a similarity among the words that will feel like a rhyme is trying to slip in there somehow. Here's the first stanza, so you can see what I mean:

“O where ha’ you been, Lord Randal, my son?

And where ha you been, my handsome young man?”

“I hae been at the greenwood; mother, mak my bed soon,

For I’m wearied wi’ huntin, and fain wad lie down.”

Each stanza after that has different words as it continues telling the poem's story, yet each still has lines ending in the final words "son," "man," "soon," and "down."

Because "son" and "man" look alike and do end in the same final consonant sound, even though they don't share a vowel sound, we'll call them "slant rhymes," which are almost rhymes. 

If you happened to have a Scottish accent, then when you say "down," it would sound just like "dune." So imagine also that "soon" rhymes with "down."

Now you can see why some folks would label this poem's rhyme scheme as AABB. "Son" nearly rhymes with "man," and "soon" nearly rhymes with "down," especially if you pronounce the vowels the way a Scottish person would.