Major bummer. I truly do feel for you.
My stepson, who is now 16 years old, was like that when he was that age. He went to counseling to deal with his feelings regarding his parent's break up and the partial absence of his father (not living with him all of the time anymore).
He STILL acts up at the age that he is now, but in different ways...it's like a residual thing.
Another thing that we did was that my husband started spending some one on one time with each of the kids. It's hard to do that with everything else that has to be done in a day, but a 30 minute walk through the neighborhood to have a chat can make a world of difference to a kid.
Here is one more thing that I found out, but it took me awhile to get it. Even though the behaviour was directed at me, it had absolutely NOTHING to do with me personally. It was all about his parents and getting their attention. I was just a safe/easy target.
Try to find a way for your husband to let him know that you are off limits for disrespectful behavior. Believe you me, the behavior wont go away...if he is allowed to act disrespectful towards you now, then he will always think it's okay to do that.
You are right in both that you shouldn't have negative feelings towards a five year old kid, but it's also understandable when you are getting the brunt of his ill behaviour!
Hang in there & it does get better.
What worked for us was counseling & one on one time with the bio parents.
Just my two cents.