You have THREE kids younger than he is?
I'm with the others - RUN, don't walk away. There's a reason that social workers usually do birth order placement ONLY (adopting kids younger than your youngest child) - it's not just to maintain family identity, it's also to protect the younger children when and if things go wrong.
And with this kid's history? You can have all the alarms you want, it's not enough.
There are tons of kids out there who need homes and a loving family. But again, I don't believe this is the one for you. And shame on his CW for being dishonest about his true issues. Your CW is right, this is too much - not that you're not great parents, but that in the big picture, this is NOT the match for you.
I'm sorry, but you must put your other children first. Taking this boy will put your entire family in a world of hurt - physical and emotional - that you never imagined.
But don't give up! Just keep looking, your new son or daughter is out there.
,
Sandy
__________________
Proud foster mama of many;
Proud transracial adoptive mama of:
J, age 9-1/2, and Q, age 7 (OMG!!!)
Still hoping for more kids..... 
Nellie (the cat), adopted stray
"Friends are the family you choose."