I say, and with God's success: If he performed ablution with the intention and then converted to Islam, his prayer with this ablution is valid. Similarly, if he performed ablution without intention and then converted; because the intention of a disbeliever is void due to lack of eligibility, and intention is not a condition for ablution. Refer to: Sharh al-Wiqayah p. 108, Al-‘Inayah 1: 115, and Radd al-Muhtar 1: 165, and God knows best.