A confession absent corroboration is generally not sufficient to establish probable cause.
If the victim does not allege the act occurred, then the DA will have a hard time making the case.
If I went in to the local police department and confessed to a murder, even if the alleged victim were missing, I could not likely be successfully prosecuted absent something more to tie me to the crime.
The same holds true here. He may admit to it, but if the victim did not say it happened, it's tough to make the case. Therapy can reveal "repressed" memories, but these are easily attacked and are very vulnerable to suppression as the techniques used are far too often suggestive and inconclusive. It's kinda tough to allege that the victim never alleged the defendant did THIS to her, then suddenly - after "therapy" she comes out and says, "Oh yeah, he actually did do THIS to me." That sort of after-the-fact claim is very vulnerable to counter arguments.
Now, while the DA may not be willing to prosecute, you can always consider a civil suit against the abuser. The burden of proof is less in a civil suit.
- Carl