Trust your mumma instincts.

It's not always the best idea to persist with day care. Having worked in day care myself for 5 years I saw that some children were simply not strong enough to be there... but sadly their parents had no other option. It can also be very expensive paying for so much unused daycare which makes it hardly worth it.

I would err on the side of caution and pull him out at this stage, like any other kind of immunisation (which germ exposure is) sometimes it's better to let a bit of time pass until they are stronger.