In the sentence: Il était très maigre comme le sont certains fous rongés par une pensée malade, there is a literary inversion.

Instead of writing: comme certains fous le sont, the writer puts the verb first so he can ADD right after the noun fous, rongés par une pensée malade. Had the writer not done this, he would have had to rework the entire sentence.

The plural “sont” is used because its subject is the rest of the sentence (there is an inversion). Since the rest of the sentence is plural (certains fous…), the verb needs to be plural.

The “le” refers to the state of being very thin (très maigre).

Basically, the author is trying to define exactly how thin the patient is by comparing him to the state that these “fous” would be in. A very basic way to phrase it would have been:

Il était très maigre comme certains fous rongés par une pensée malade sont très maigres.