Regression is a coping mechanism that people (of all ages) use to deal with something stressful. Is there something about the school he doesn't like? When my DD went to a school that wasn't treating her well I had to battle her every day to get her to go in the classroom. Now that she is attending a school that is nice to her we don't have any issues at all.
Has he recently had a new sibling, or is he with babies at the day care, many children do this to get the same attention they see others getting more often than not when mom has a new baby they are emulating with baby talk . Have you sat in on the day care are the children stimulated enough?