My kid is 6 year old and suddenly he has become very clingy. He feels very insecure without me. What could be the reason?

Talk, talk as much as you can. Try to know if he is disturbed, annoyed, sad, afraid of something! Kids show mood and behavior change when something is going on in their mind and they are not able to convey. Don't get paranoid in front of him. Be calm and in the course of normal conversation ask him about his day-to-day activities in school, play area, friends, teachers, relatives, your friends, etc. Eventually he might speak up.If you feel his behavior changing a lot, you can seek a counselor's help.