It would do no harm for couple of days, and he can have yogurt daily.
You can use gel or mouth paint containing nystatin or clotrimazol 3-4 times daily over affected area.
Is this gel available OT?
So the thrush should not get too much worse untreated today?
You should inquire, the product will be available without prescription. No harm if left untreated couple of days. Though it may slightly increase in extent.