Although the illnesses may be distressing, which would you prefer: potentially being sick now and missing a few days of daycare or potentially being sick in elementary school and missing out on precious learning time?
Our baby has 'moved rooms' three times due to his age at his daycare. Like clockwork, he gets sick about a week after the switch. It's never anything major - mostly a little cough or a runny nose. After that initial period, he rarely gets sick. And he was formula fed too - go figure!!
What I would be most concerned about are the illnesses your son is getting and why on EARTH your daycare center is allowing parents to bring their child to school so sick! Kids are going to have runny noses and feel a little worn down; that's to be expected but it sounds like they need to do a better job of enforcing their 'sick policy' (fevers, diarrhea, vomiting, runny nose if not clear, etc.). I'm fine with the concept of daycare, but my biggest complaint are the selfish parents who bring their super-sick kids to daycare when they really ought to be home.