In response to your questions it is unfortunate that the date scheduled for the PLO review meeting clashes with your son’s sports day, but it is important that you attend, perhaps you will have time to get back to your son’s school or ask his class teacher to enter him in events that start nearer the end of the day.
The letter you have received does not make your partner a ‘party to proceedings’ that can only happen if/when an application to court is made. It is important that you both attend the meeting and each take a solicitor with you or if as you mention there has been a breach in data protection because your partner does not have parental responsibility (PR) for your son, contact the social worker and ask the reasons why he has received an invite. The Law Society has a database of solicitors, some of whom specialise in children’s law. If you have not already done so, please have a look at our advice sheet Care (and related) Proceedings.
I hope this information is useful.