Hi OP, part of my job is to go to nurseries and train them in doing WellComm screens. Though it’s a screen rather than a test, children could feel they are being tested and it is possible to have typically developing children score red in the wellcomm simply because they are shy on the day.
However, there is absolutely no harm in referring a child to speech therapy for a more in depth assessment. A speech therapist might decide on meeting your son that he doesn’t need any support, but if he does then better to have it as early as possible.
Nursery will not be able to refer him to speech therapy without your consent. Please don’t worry or feel stressed about it. A high proportion of children score red or amber in the wellcomm, what it’s really good at is picking up children who might otherwise go under the radar, which is good because most of these children flourish with just a bit of extra support.
It might be that nursery just want to do a bit more targeted language support with him. Again this support will do no harm even if he doesn’t need it, it will be things like stories, rhymes, talking tables, and lots of modelling language through play. Good stuff for all children!
Definitely chat to them at parents evening and see what they are suggesting. You can of course ask for no more wellcomm screens if you are worried that it is distressing for him but it is a very helpful tool. Feel free to DM me if you have any more questions.