Sounds like an allergy to certain food colour. Red dye is a more common allergy (which is also in some other colours). I would try giving him the white freezies as they would have less colouring. Keep the food log. Check him head to toe for a rash a few minutes before pick-up and report the findings each day so they know you are looking.

Allergies do and can show up at random times, with something that was previously perfectly fine for the child...so move forward cautiously until you pinpoint what is going on. If you want maybe ask the family to take a picture of the rash if it appears again so you know what they are seeing since you've yet to see it. Tell them it will help you identify it when you see it and to know what you are working with.