I'm wondering if this boy could be the boy mentioned in David Brooks' confession as being killed "just before Wayne Henley came into the picture", whom it "really upset Dean to have to kill this boy because he really liked him." He also claimed the boy was killed "about two years ago." The boy was picked up on Eleventh and Rutland, and was killed solely by Dean Corll in the house on Columbia (915 Columbia St., Hacadena Heights).
The confession was written on August 9, 1973, so that would place the victim's death in mid-to-late 1971. That would work for the post-mortem interval for Swimsuit Boy. In any case, Swimsuit Boy was almost certainly killed after November 17, 1970, because that's when the boatshed he was found in was starting to be rented out by Dean. If he had been hitchhiking, and was dropped of at Eleventh and Rutland, that would explain why David doesn't know his identity despite having known most of the boys he lured to Dean's.
David Brooks also claimed he thought he recalled helping bury the boy he mentions, but can't recall where the boy was buried. Considering 17/28 of the known victims were buried in the boatshed (where Swimsuit Boy was buried), I think it's the most likely that the boy was buried there.
In any case, Wikipedia notes that the above boy's identity is "unknown", which could either mean he was identified and David did not make the connection between the boy that that was killed and the missing boy, or that he is still unidentified. Part of what makes me believe in this more is David's memory was pretty sharp and clear, and he seemed pretty capable of remembering who was killed and when.
