You mentioned that he has had medical issues since he was a baby. He sounds like a child with an attachment disorder, and you would probably find a lot of validation in reading about attachment. Here is a link to some basic information: http://psychology.about.com/od/lovea...tachment01.htm

All infants attach to their caregivers. They have to in order to survive. Some form secure attachments, and others insecure. The requirements for an infant to form a secure attachment depend on both the infant and the caregiver.

1. The infant must be able to communicate a need (hunger, discomfort, pain, lonely, etc.).
2. The caregiver responds appropriately to the need (feeds if hungry, vs. insists that baby takes a bottle when baby actually was cold or lonely).
3. The infant is able to accept the response and be soothed.

Babies with medical conditions have significant challenges for forming secure attachment. They may be in the care of various medical personel, which makes attachment to parents or any specific caregiver difficult, and subject to being disrupted. They also probably experience a lot of pain, which no caregiver might be able to soothe.

Children with insecure attachment styles do not feel safe and comfortable. The world is a scary place for them. They often act out a lot, sometimes in scary ways. It isn't easy to treat attachment disorders, and can take a long time. The child has to learn that he can trust his caregiver and be soothed and comforted, have his needs met, and be safe and secure. And they challenge and reject efforts to teach this, because they can't trust it and will put you through a lot in order to test the relationship and see if they really can depend on you, because they never have felt secure or learned to accept love. Progress is often slow, but this is treatable when it is understood. The most important thing is that you can stick to it and be steady and dependable even when you are sorely tested. It doesn't mean giving in to all his demands, but it does mean setting clear limits and being very consistent.

I wish you the best.