4 and a half year old pit with a whining problem

Question
We have a 4 year old pit bull named Enzo, who in the last year has developed a serious problem with whining. He will sit next to the doors and whine, laying on his bed, he'll whine, and even in "his" chair in our room. We will let him out back, and 2 min later, he is next to the door wanting to come in. We let him in, and he goes next to the front door and whines. We let him out (our house is gated) and 2 min later, he wants back in again. This will continue all day. His cries to come back in sound as if he is a dog being tortured. And it just gets worse if we do not let him in the second he starts. Friends and family cannot believe such sounds are coming out of a 100 lb pitbull! He sounds like a dog attached to an electric chair! It gets rather frustrating. We've tried opening the door, telling him no, closing it again, and then letting him in after he has sat quietly, but the problem is, he won't sit quietly!  
 A bit of back history: we've had him since he was 5 months old, he was our "fur-st born", and now we have a 1 year old son. I used to run a few miles a day with him, but since having the baby, unfortunately, Enzo is too strong and has leash aggression, so I'm not able to take both him and the baby running by myself.
We do still take him out on walks, but even that does not stop him. I've even left the gate open between the front and back yard so he has free reign, and he STILL wants to come through the house and make us open the doors. We do play with him, and will take him to dog parks, but nothing we know how to do will stop the whining and crying.
I'm getting so fed up that I don't know what to do. Any suggestions? We've thought about a shock collar, but have heard mixed reviews about using them on Pits. We are desperate, as his crying wakes our son, and makes me feel like I have 2 infants I have to deal with crying!

Answer
Because of the variables in this issue I would strongly recommend having a behaviorist/trainer come to your home.  Without seeing his body language it is difficult to truly determine a cause for this behavior.

It sounds like a start would be some more challenging exercise such as flyball, agility or tracking so you can tire his mind as well as his body.