We are a Reformed discussion board. We don't believe that an individual is in charge of his baptism. It is a church ordinance. It is up to the elders of the Baptist church to decide whether individuals should be baptized or not.
That said, you probably have not provided enough information for anyone to answer your question. For example: How old was this individual when he was baptized? Was he baptized upon a profession of faith? In what kind of church was he baptized? Who did the baptizing? Etc.
No. For one, we all sin and we don't get baptized again every time that we do. Why does he feel it was illegitimate? Was he consciously lying at the time? To me the fact that he repented after discipline and "returned to the path" would be a sign that he was saved beforehand, not that he never was.
If one was an unbeliever at the time of baptism, it is not truly a baptism. Therefore, no true baptist would ever support "re-baptism" but only one proper baptism (i.e. believer's baptism, and that probably by immersion).
Coming under discipline isn't enough to prove an individual is unconverted. If there's no doubt that beforehand he was unconverted, and he now truly came to faith for the first time, then he should be rebaptized.