Talk directly with the caregivers and ask 'so, what happens when (son) uses the toilet?' You might find out a number of things.
You don't say how old your son is, but some children really do not like anyone but their parents to wipe them. If the children use the toilet more or less independently at daycare, the caregivers may not know when he has pooped. Or maybe he doesn't want anyone else to wipe him, so if they ask him "did you poop?", he might say no to avoid it.
Also bear in mind that there may be any number of children using the potty at any given time, depending on the construct of the bathroom. If another child is having a hard time with pants-messing or needing more attention in the bathroom, the caregiver may not remember to check in with your son.
I have taught preschool for quite a long time. This is why I suggest these scenarios. I've worked in daycares that have had one potty in the classroom and those that have 3 stalls across the hallway. Plus, if you have higher-need kids or a substitute teacher or a squeamish teacher's assistant who doesn't like doing it-- or a kid who doesn't ask for help (because we are trying to help them become independent) then all of these things factor into the situation.
So, do talk to the caregivers. Let them know, directly, that he had a previous UTI and you are concerned about his getting clean. Do offer the wipes if need be, but remember that other kids might use them too. There are a lot of kids to keep track of, so keeping track of your wipes isn't going to be high on their list of priorities. But if everything else at the daycare looks good, ask your husband to cool off a bit. Many people would love for this to be their only problem with a daycare.