If he rejects the baptism, the baptism was useless. If an adult becomes a believer, he is baptized as an adult. Baptism does not save anyone. If one argues that not all infants that are baptized later become Christians, this adds no bearing to their stance since there are adults that are baptized and then later reject Christ. Similarly in the Old Testament, Esau and Ishmael were circumcised, but it was useless.
Infant baptism reflects the message of the gospel.
Baptism is the mark of God's prior love to us, which comes before our response. It is God's love that calls forth our response. Baptism is not a witness to faith but is the witness to grace. The infant baptist side more closely resembles what God has done for us. Likewise in Acts where 3000 were baptized in one day, there were bound to be some children who were baptized as soon as their parents believed.