What's the age? Since when ? Is it on purpose? Or when he is inattentive? Some have abnormal control over convergence and it is not disease. However getting a eye check up is not a bad idea. Another cause can be of a stress nature. Symptoms may be particularly associated with close work, like excessive use of gadgets,  mobile etc and may include frontal headache, ocular fatigue, blurred vision and double vision.

Home check is not a good idea. You better consultant an ophthalmologist in clinic. He may have to be checked after putting some drops in the eyes.