Enquire about his activities at school . If he cant pay attention there and involve in disturbing other children as well , it may be suggestive of ADHD. There may be other reason as well. Try to note his daily activities and about his company. Some time company of bad students also may destract your child from academics. In both condition he needs proper counselling and behavior therapy. So consult with a psychologist  and start medicine for  him if its recommended.