If he is arrested, that will go on his record. If he ever wants to go to America for example, he will need to declare his arrest when he applies for an ESTA. An arrest in itself shouldn't need to be disclosed for anything else, but there are certain jobs that will ask about it.
If the case ends up not going anywhere at all, what we call no further action, then that will be it, he wouldn't have to disclose it other than what I've said before. There can be many reasons for no further action. It will be the cps (crown prosecution service) that make the decision, and is usually down to lack of evidence. This doesn't mean they don't believe you, but that there isn't enough evidence to take the case to court.
In order to receive a caution, he has to admit what he has done. I can see this being something that could happen for harassment possibly, but not rape, if he is also dealt with that. However, it's not beyond the realms of possibility. A caution stays on his record for ever, although most job applications etc only ask if you have received a caution in the last 5 years. As well as admitting the offence, the person has to gave no other cautions for similar offences.
The other possibility is that he is charged, and the case goes to court. This would involve you giving evidence at court, but measures can be put in place to protect you such as giving your evidence behind screens or via video link. If found guilty he would have a conviction which would always be on his record and need to be declared.
Name, if he is ramping up, please consider phoning the officer or asking to speak to another one if she has gone on leave. I know you're away this weekend, but if he is really pestering you and you're having to claim you have cystitis, I'm not sure this can wait until the 13th.