Nothing. The evidence would be based on forensic testing, not the taste.
Yeah, forensic testing. I'm sure the crime lab would be all over this one, to substantiate a simple battery charge. They'll make sure and isolate a sample from the garbage, then compare two DNA samples.
A defense attorney would absolutely try to dredge up all sorts of shit to discredit the only material witness. The only way to avoid it would be for the prosecution to find better evidence (DNA, which is unlikely for a simple battery charge, or another witness coming forward who saw the guy do it) or not put on any evidence that the witness knows what semen tastes like. Do you think the prosecution is going to find a cooperative witness, willing to do that?
This won't go anywhere. What might are the guy's sex charges. That's quite a bit more serious.
EDIT: I'm thinking about this, and I'm really not sure how many simple battery cases actually see a trial. Far more likely it gets pled out or dropped. Probably dropped if those sexual contact with a minor charges stick.