Adoptive son attachment issues?

We took our youngest son into care when he was a little over 2 months old. We have adopted his older sibling so he was moved to our home from a foster home where he had been for a little over 5 weeks. He was abandoned at the hospital with various drugs in his system to a biomom who admitted to drinking and smoking during the pregnancy as well as using marijuana, meth, pills and cocaine. The first two weeks he was with us he was a very relaxed baby, but after those two weeks the poor thing cries to the point he turns red in the face, sometimes almost purple. We were told it's colic and he would grow out of it around four to five months old...well, he has not. We have tried baby wearing, snuggle bags, weighted baby bears, even changed the formula to a colic type. I was wondering could he be suffering from separation anxiety? Does he miss the foster family? I use to think he hated me but now it seems he is overly attached and I have been told I held him to much leading to spoiled baby syndrome. When he sees me or I pick him up he stops screaming almost instantly. There are no tears with this screaming, just a high pitched whail.What am I doing wrong? I feel I am bad mom since I cannot comfort my baby. His older sibling was never ever like this and that child was very sickly when we got that placement.