YANBU to want an appointment- but go in from a 'how can we stop this happening again' point of view, rather than a 'omg how dare you not notice he was ill' sort of way.
For a start, does your DS have a healthcare plan in place? Not sure if it's compulsory in preschool, but at school every child with a health condition has one, with signs of emergency and what to do clearly explained.
For that matter, are the preschool fully aware of his health problems? Have you perhaps handed out fact sheets so that they know he's not just one of these children that catches every cold going, but that he has a diagnosed and documented health condition?
Are you sure your DH relayed the message correctly, and that the member of staff heard and understood the message? It's easy to play Chinese whispers when asking someone else to pass a message on. Perhaps it would be better to relay messages in writing in future- maybe via a home/preschool diary?