It's ok! Some preschools take children as young as 2 and they are not potty trained. Maybe it would be easier for him to use wet flushable wipes while at school. Kudos to you Mom for even getting him to wipe! My son didn't want to do it for fear of getting his hands dirty. Speak with the teachers and ask about their policy. They may not mind helping make sure he is clean.
I working on the same thing but making him do it no matter what
Just emphasize mom doesn't like doing it and the longer he waits the harder it is to get off or make him use wipes for when he waits to long