I don’t knowwhat he wants. The underlined clause is a/an-

A noun clause is a sub-ordinate clause used as a noun in the sentence.
A noun clause may be used as a subject or direct object of the verb, as a predicate noun, as object of the preposition, or as an appositive.

In the given sentence, the clause ‘what he wants’ serves as the object of the verb ’know’.
So, it is a Noun clause.