J.B.
Hi R.,
to be honest, I've never had to deal with a situation like yours.
But it does sound like an "attachment" issue at the heart and core of the issue, as your son's ability to bond has been messed up due to the abuses of his bio mom.
I'd like to recommend that you look into Baby Wearing, or Attachment parenting websites - there are lots of them. I'll include a few links I found that I think might be helpful and you can research on from there.
I "wore" my baby with a baby sling for actually probably a year or more once he was about 6 months and my c-section wound healed up where I could tolerate carrying him. He was much more peaceful and less fussy once I started carrying him with a sling (as I wasn't very strong elsewhere, and he's my only child).
http://www.attachmentparenting.org/princtouch.shtml
They are talking about babies here, but even twenty two months old I would hope he'd be able to respond eventually. Time does help heal wounds.
Another one I thought of was websites having to do with adoption issues - because, in a real sense, you are adopting your stepson. The process is complicated in that he "remembers" his dad but he's dealing with a "new" mom, even though technically he has known you all his life (so far, 22 months)...
http://forums.adoption.com/korea-adoption/325155-help-bon...
So some of this is specific to foreign adoptions and can be ignored, but eventually I'm sure you'll hit on something that resonates with you on your situation - and I thought it interesting to notice that many of these parents also used baby wearing slings and wraps to carry their child.
The Bjorn is one that is seen a lot at places like Target or Wa-Mart and is cheaper but I found them very uncomfortable being a heavier mom myself. So you'll have to take consideration of your body type and experiement - maybe attend some LaLeche League meetings or find a local adoption support or if you are really blessed, a local shop that carries different types of baby wearing systems to find one that works well.
Another thing that I found helpful with my son was using lavender oil from the healthfood store - this was actually something I learned from the "All Creatures Great and Small" animal veterinarian series by James Harriott (sp?) years ago - and maybe he got the idea from watching his grandma, I've no idea, ha!! but basically I would rub just a drop or two of lavender oil around my son's chest and nose so he could smellt the lavender, and I'd rub some on my chest (since I was breastfeeding at the time but it works even now, if he's really fussy!!) - the idea being then that he'd "smell" that I smelled like "him" and didn't smell "different"...and so it seemed to calm him down. Plus lavender has calming effects anyway - well known as one of the safer herbs around babies and young children.
So anyway I'm sure this is not going to be a "one tap of the magic wand" miracle cure but I'm hoping something here will click and work for you.