Yes, he's playing baseball and he's in Cub Scouts. He has lots of friends and gets along with them.
When I tell my son "no" he argues and whines to try to get me to bend. Most of the time I make a deal with him or stay with the "no" answer. When we are getting ready in the morning and I leave him in the bathroom to brush his teeth, he panics and hollers my name. He's got to know where I am in the house. Even if it's during the light of the day and he can't find me in the house, he panics.
You are the center of your son's life and he is having anxiety about your absence because his dad is no longer around. He does not want you to disappear. He fears being all alone. Simply tell him that he need not worry. You will make sure that he knows when you are not around. Have you asked him why does he become so fearful when he is alone? What really makes him afraid? If he cannot tell you, then have him draw a picture and explain his picture. He simply wants to feel an attachment to you even in your physical absence.
As for giving in when you say "no", try to stick with your word. Ignore his whining because you are the boss. It is not necessary to make a deal with him when you are trying to enforce something. He needs to learn that you have the authority to discipline him and obey you without trying to manipulate you.