Who provides the catering at the school? If the kitchen staff really didn’t verify/know who he was before serving him, then this could be a breach of their procedures by the kitchen staff. It depends on what the kitchen staff actually did when they served him, they should be aware of all children with dietary needs and have access to that info in some way. It may be a case where kitchen staff are very familiar with the children at the school and are well versed in which children have dietary needs and which ones don’t, even newer children now it’s a few weeks into the new term. Some kitchen staff put a great deal of effort into getting to know the children and their different needs.
You are right in that although your dc doesn’t have dietary needs, another child might and this seems like it could be a risk factor that needs to be addressed.
If the provider uses an epos system, the child’s name should be selected and their dietary info should come up (provided parents have given the info to the school/catering provider, you’d be surprised at how many parents don’t do this or give confusing/wrong information). If they are using a paper based system they should still be checking some sort of list to verify any dietary requirements.
I would say it’s worth mentioning to the school, failing that reach out to the provider. Your child was fine on this occasion, another child might not be if it happens again. Kitchens have to follow HACCP procedures to control and eliminate risk factors. In my view unless they were confident they knew your dc’s name, they haven’t done all they can to control/eliminate the risk here.
Also just curious, why wouldn’t your son get a free school meal when he falls within the age range for universal infant free school meals?