I agree that the OP ideally needs to get the teacher or school’s side of the story before making an official complaint. It is possible that there was some miscommunication and, especially, since he was feeling unwell, maybe the son misunderstood what was said to him? Or maybe not, but it would be handy to try to check.
The comments that the OP’s son is not a child….well, it seems that in England a child is defined as anyone who hasn’t reached their 16th birthday, so yes, in legal terms, the OP’s son is definitely a child. He’s not a small child, granted, and is presumably capable of managing responsibilities and taking care of himself (to a point), but he is a child.
I think some people also don’t consider that age isn’t always an indicator of capability/maturity. People are individuals. They differ. Some 16-years-olds may be very grown up and streetwise and capable, other 16-year-olds may be very “young” 16-year-olds (if that makes sense.) They might not be as confident, may still be somewhat childlike in some respects. Some can be naive and inexperienced.
When you’re away from home, in a different country, for the first time, and you have stomach pain/vomiting, it seems natural to me that you’d feel vulnerable, especially if you happen to be by yourself at the time. From the OP’s posts, it looks like her son did try to contact a teacher at a number (a WhatsApp number?) with which he was provided. Perhaps before he called his Mum, perhaps afterwards, but he did take steps to get in touch and let a teacher know. It seems perfectly reasonable to me for him to also call his mother. He wasn’t feeling well, he was by himself, perhaps he felt at a bit of a loss, uncertain…it’s surprising that some comments seem to be more or less mocking him.