Then he should plead not guilty. The crown will have to prove their case and from what you say, there is little evidence to support the allegation of assault. By it's definition, harassment suggests repeated behaviour. One phone call does not constitute harassment. Swearing on the telephone does constitute an offence but I'd be surprised if that isolated incident would interest the CPS since, it does not appear to be in the public interest to pursue a prosecution.
Then again, you're a mother. And mother's have a tendency to steadfastly support their boys. Are you sure you have all the facts?
The Polish girl may not be a particularly nice person but she has done nothing wrong legally. The police must have some evidence that your son assaulted this girl and that the phone calls (after the separation) have been offensive and repeatedly so. Otherwise there is no point submitting a report to the CPS that is doomed to fail. Because without evidence it certainly is.