Hi,
Is he separated at all from you during the day at the moment? I know that both my boys struggled when we had our first separation when they went off to Nursery at 3. At this time they both became quite anxious, our younger one even developed a stammer which we were able to sort out after a couple of months thanks to advice from the Head.
Is your son anxious about anything in particular? If he has a large vocab is he able to explain to you what is upsetting him? Hopefully someone else here might also have some ideas...