Dear M.,
It's my guess that your son may not be as well adjusted to the "split" and attending pre-school as you may think he is. Not to say there is anything "wrong" with him, but my daughter did exactly the same thing at 3 years old when she was going back and forth between me and her father....and attending pre-school. Reverting like that is usually a symptom of something, and it may be that he really just wishes he could just go back to being a baby, when things were easier for him. Or, it could be stress that he himself doesn't understand and can't vocalize. I certainly wouldn't do punishing just yet. If you and your ex have a fairly decent relationship, and it sounds like you do, then the two of you need to talk about how you can make transitions from being at one house to another easier for him. You may even want to find a counselor that deals with young children of divorce who can give you some tips and talk with your son about how he is feeling. My daughter went, and "play therapy" revealed a great many things that she couldn't outwardly express. For instance, she felt like she was in pieces. A piece of her at mom's, a piece of her at dad's, a piece of her at school. We had to make sure, in all of it, that she felt like a whole little person. That she wasn't just a thing that could be split into pieces. After we recognized that, things got better for her. And another thing I learned from counseling is, never ask a child that age "Why?" They ask US that 50 times an hour......but to ask them that same question and think they can answer is not productive. "Why are you crying? Why are you throwing a fit? Why are you pooping your pants?" 99% of the time, they don't KNOW why.
So, we have to figure it out for them.
Your son will be okay, I am sure. But do look into having him see someone. Your pediatrician, even the pre-school, may be able to give you some references. Do it now so you can deal with it and help him move forward.
I hope this helps.