Ask the school counselor to observe your son in the classroom. The counselor can give you an objective opinion of what may be going on with your son. Something as simple as moving his desk to another part of the room or setting up a secret cue such as the teacher touching his shoulder may make a big difference in your son's attention.
If your son is in an upper elementary grade and you haven't already done so, you may want to talk with your pediatrician about what is going on with him. The doctor may have other suggestions for things you could try.