Hi C.,
I have to agree with the woman who said to just nicely bring it up and see what happens from there. It will be awkward but it must be done and you'll feel much better afterward. You may even preface the conversation with a "this is a little awkward BUT..."
My PERSONAL experience was that I much preferred a Children's Center vs home day care. The regulations are stricter and I get full reports (when diaper changes occurred, what was in the diaper, what, how much and when they ate and when and for how long they napped). I honestly don't know how daycare providers do it - the adult to child ratio is crazy. My preference is not intended to negate the incredible work that in home providers do BTW. Just sharing...
It is really common for the babies to get sick a lot the first year in daycare. There are just a ton of germs floating around but it gets MUCH better as they build up immunities. I wouldn't worry about that too much unless you suspect toys are being washed on a regular basis.
Lastly, to venture away from what the other moms are saying. If you son is coming home with dirty socks that is strange BUT at least you know he isn't being placed in a bouncy chair and left to entertain himself. To me, that indicates they are involving him and letting him cruise around and if it's socks then they are holding his hands and helping him. My son could stand while holding onto things by 4.5 mths, maybe yours is doing the same?
Okay, off the soapbox. Best of luck to you!
T.