The decision what he has offered is positive.
 The decision about his offer is positive.
- Do you mean, that he has offered.
- If not, then I'd parse it as (that) what he has offered, a relative clause.
- relative clauses begin with a relative pronoun. The phrase about his offer begins with a preposition.
- noun clauses have a subject and a verb. The phrase about his offer doesn't have a subject or a verb.
Student or Learner