My son started judo just after he turned 4. I would say that it would really depend on the patience of the teacher and if your son was able to concentrate for the duration of the lesson. They have to be able to participate not just run around the class doing their own thing. I was surprised at how my son could. Speak to a professional teacher as every child is different and your son might be ready to start. You can always have a try out.

Good luck