His three friends lifted him up and all he had were bruises and cuts. I had to read this sentence in a story. I'm confused with the use of 'were' in it. If the word 'all' in the clause 'all he had' means the only thing, I think it should take a singular verb. Please explain to me.
First of all, I believe that sentence is badly worded and not a good example of English (even though it is technically grammatical). This sounds more natural and is clearer, "His three friends lifted him up and his whole body was covered with bruises and cuts." The sentence as written is better suited for cases of actual possession/ownership (e.g. "All he had left were bananas.")
Regarding your actual question, whether "all" is singular or plural depends on the number of what "all" refers to.
All the trees are beautiful (all refers to trees which is plural so all is plural as well)
All of the honey is gone (all is singular)
All he had was one thing to say (all refers to thing which is singular)
All he left to his name were failures (all refers to failures and is plural)
His three friends lifted him up, and all he had were bruises and cuts.
Yes. It means the only thing he had were bruises and cuts.
A comma is needed in this Compound-Complex Sentence. This type of sentence has two independent clauses and one or more independent clauses. Yours has two independent and one dependent clause.
His three friends lifted him up | and | all |(that) he had | were bruises and cuts.
Independent: His three friends lifted him up
Independent: all were bruises and cuts.
Dependent: (that) he had = this clause is introduced by the relative pronoun "that' which is left out but understood. "that" introduces the clause and its purpose in the clause is a pronoun with the antecedent (that word the pronoun stands for) "all," another pronoun. [remember a pronoun takes the place of a noun or another pronoun], and to be the direct object of "had."
(that) he had = he had (that) = he had all = subject | verb | direct object
all were bruises and cuts = were is a linking verb, and without going into that topic, the verb links "all" as a pronoun to bruises and cuts which are both used as nouns [a noun or pronoun is called a predicate nominative that the linking verb is linking to the subject] to "identify" or "explain" the subject "all."
all = bruises + cuts [Sounds like he was beaten up and robbed of his possessions, and his friends were picking him up off the ground, or he lost the fight, and instead of victory and pride, it was cuts and bruises]
So, together, the dependent clause shows that he possessed "all" and the independent identifies or explains what he had: all he had were bruises and cuts.