It kind of does sound like he's on the autism spectrum, and maybe that the customer service thing is doable because it's more scripted and set.
I think the first step is to sit down with your husband and really hash out what he would like his relationship with his brother to be. Once he knows that, does it match with what you want it to be? If not, get to a compromise point. Then lay it out to the brother, being open to compromise if he suggests something reasonable. And hang on to those boundaries. A counselor might be helpful. See one yourself, or with your husband, or even get a social worker who will help the two of you navigate the conversation with your brother.
It's going to be exhausting. Sorry you have to deal with this. Try to be compassionate toward the brother. Maybe try to get him interested in something without you, like religion? Is there any way he'd like to go to church? Some of them meet really frequently
