If this particular day care is relatively new to your son, he may just be reacting to a stressful situation. I do evaluations at child care centers when there is a concern and I can tell you to start there and work outward. It also sounds as though the staff may not be as accomodating or enlightened (or professional) enough to deal with this. When children react to stress, their bodies deal with it, just as we do. Toiletting issues are usually very common. If no one gets all bent out of shape and recognizes this is a SYMPTOM of something and deals with THAT, Bingo! But what usually happens is the symptom (accident #1) gets blown out of preportion and there is shame, belittling, confusion, etc, and now the child is dealing with the original stress and then more stress over the accident and then more stress over the shame and on and on. Is there any wonder? And now the staff has moved it up to another level with a health concern? I'd wet my pants too! When little children come to me with a story about someone fighting or hitting them, I always start with this question? "What was happening right BEFORE you got hit?" This offers a clue to what prompted everything. Use it for your situation. What happened right before he started wetting his pants? If something new happened,this may offer a clue. Children love predictability- love it!! That's why they want you to read or tell the same story over and over and over...they already know the outcome and there is a comfortable feeling with predictability for kids. So much of their world is totally out of their control. When their reoutine is disrupted, they feel insecure and react accordingly. There may also be something going on with a new teacher or new student if this child care is not new. Ferret out info and talk with your child. I will tell you also: A child caught between a hovering micro-managing parent and an unconcerned child care provider will end up doing more than wetting pants. Medical concerns are also never to be disregarded. He may need a checkup.
I did an evaluation a few years ago at a very upscale, trendy child care center with a 4-year waiting list (out of 227 local centers, it was the most expensive also.)The center director contacted me with a concern about a 3 year old boy. He had the same issues you stated - potty accidents out of the clear blue sky. I went to the center, did objective observations (no contact with child, no interactions with staff, etc) then went back and worked a little with the child and staff and step 3 - home visit. This I always do. Family consisted of Mom (engineer in Detroit) separated from Dad 2 months, 1 male sibling 6 years old. Mom told the kids I was a new teacher at the center and wanted to come out to the house for a visit. Kids need to know why you are at their house. After a few minutes, the boys ignored me and went about their business. After a few home visits and a few meetings with center staff and Mom, we all shared information. I felt the marital separation may have precipitated the first accident which was understandable, but the day care staff may not have handled things positively after that. No one was to blame. We all wanted to understand and to help the child. Switching centers would not have helped anyone. There would be more change for the little boy to deal with and you can bet he would have felt he caused this also. (We already know because young children are so egocentric, they look at everything in the whole world as happening because of some connection to THEM. They define everything this way) this is why children feel responsible when anything happens in their family. We don't want to add to this. Also: I am not a proponent of tangible rewards (stickers, candy) for regular accomplishments - giving them something they can hold in their hand. Give them something they can hold in their heart instead. This will help them build their own self-esteem. Tangible rewards never end and they just keep upping the ante and the value keeps diminishing as time goes by. There is a true wonderful feeling inside us when we do what we know is right and good. Nurture that in your children by offering them real honest loving feedback and ALWAYS connect the deed to the compliment - no empty words, please! Not everything they do is wonderful all the time! Kids are smart! They have wonderful intuition! That's why they react so quickly to things we are oblivious to!
If you are a true helicopter Mom, you also may be very good at hovering over him at home and anticipating his toileting needs so he doesn't get an opportunity for an accident. Maybe the center staff weren't as tuned in to him which allowed the first accident and then it was downhill all the way after that. This may be a simple answer to the whole thing. Good luck and consider what the staff has shared with you and never disregard the medical thing, even if you feel it is way out in left field.