It is because his father is (ever) truthful and his mother speaks the truth, that the Brahmanas have named him Satyavana (i.e. truthful).
सत्यं वदत्यस्य पिता सत्यं माता प्रभाषते। तथास्य ब्राह्मणाश्चक्रुर्नामैतत् सत्यवानिति॥