Free verse means that there is no set pattern of rhythm or rhyme, and a quintain is a five-line stanza. There are 16 quintains breaking up this long poem.
Even though there is no specific rhyme scheme in "Daddy," there are a lot of end and internal rhymes. The end rhyme started with the first line, which ended in "do," and is repeated often, all the way to the last line, which ends in "through." The oo sound is overwhelming; just look at stanza 10, which ends in these two lines:
The boot in the face, the brute
Brute heart of a brute like you.
This poem is held together by sound as much as meaning, and rhymes and repetition can be found throughout.
Just like rhyme plays a big part in this poem without having a specific scheme, rhythm is important here even though it doesn't fit into a specific pattern. There is a lot of iambic verse, which means that the line is patterned by unstressed syllables followed by stressed syllables. Let's look at line 1 as an example (the stressed syllables are bold and italic):
You do not do, you do not do.
While this iambic rhythm is not carried throughout the poem regularly, it pops up every now and then, making this feel lilting and rhythmic, but not over-the-top singsong-y.