Basically, I think Weevil is Whitebeard’s imperfect clone. We know cloning exists in One Piece, considering 99% of Germa’s population are clones. However, who’s to say they’re the only ones who can do this? As we’ve gone through the New World we’ve been introduced to many scientists and engineers who are highly capable (Vegapunk, Caesar, Judge). Bakkin could have reached out to one of these guys or someone else entirely. Perhaps Bakkin herself is a capable scientist and cloned Weevil herself.
Most people assume that Weevil isn’t Whitebeard’s son because it’s clear Bakkin is manipulating him to get Whitebeard’s wealth and besides his hair, he looks nothing like him. It also wouldn’t make sense for Whitebeard to have a son and leave him behind or for Bakkin to hide his child from him, since ya know she wants money. This also leads to another question, why Bakkin waited until after Whitebeard’s death. Obviously the answer would be because Whitebeard would confirm he and Bakkin were never together.
However, all of this evidence against Bakkin’s claim is so overt, it’s clear Oda only WANTS us to think that Weevil is obviously not Whitebeard’s son, and it fits Oda’s style. Make something seem too outrageous to be true and then confirm it. Yet Whitebeard and Bakkin having a child and Whitebeard leaving doesn’t make sense either. So the best way to reconcile Weevil being Whitebeard’s son, and yet not really, is for him to be a clone.
Weevil has surgical scars running around his arms and neck, suggesting that he could have been put together. This could be because he was imperfect as a clone, and further surgery was required to complete his creation. This also explains why a fully grown extremely powerful son of Whitebeard was unknown until after his supposed father’s death. He was cloned after Marineford, when Bakkin saw an opportunity to take everything Whitebeard had acquired.
Remember Germa’s soldiers grow much quicker than a regular person, and the even faster rate of Weevil’s growth could have resulted in his imperfection and strange appearance. And the DNA needed to to clone him would have been relatively easy to get, especially after Marineford. With how small she is, I can see Bakkin stealthily sneaking to Marineford or Whitebeards grave to get a bit of hair or blood.
Weevil is technically Whitebeard’s son through cloning and Bakkin’s trying to use him to get Whitebeard’s supposed riches.