I know in our state they encourage you to go to DCF and read the kids file, that could give you a little more info if you haven't done that. Also if you tell the case manager you think it would be helpful to know more maybe he could give you more info.
At our DCF training they said that the majority of children in foster care have experience some degree of sexual abuse. One case manager said that she could only think of a few kids she had worked with who had not been sexually abused to some extent. Also keep in mind that it might be that he could have had adults around them that were not appropriate with their sexual behavior with each other, or he could have been exposed to porn, or to inappropriate sexual comments, things like that that you might not think of. It is so sad to think of but the sad reality is that there is a high likely-hood of sexual abuse, just statistically. You probably do this all ready but make sure that he is not alone with other kids even for a second, it is normal to be curious but make sure the other kid is safe and not feeling like her privacy is being invaded, actually I think I would keep him in another room when she is going potty/getting changed, just to be safe for her.
It sounds like you do have a good relationship with his case manager and again, a good case manager should really be able to help you with how to talk to him about these things, they should know how to bring up topics of sexual abuse as they are trained for it and have dealt with it many times before.
I don't know much about the kinds of trauma he has endured. The caseworker hasn't given much information. He did tell me that there was alot of physical violence. And that his biomom was given a week to clean what I consider normal daily stuff: Dishes, vacuuming, picking up toys, laundry, moping, sweeping, etc. And she couldn't do it. So from what I gathered the home was disgusting. He has opened up about his moms ex pushing on the back of his head really hard and smashing his face off of the wall repeatedly. That's about all he has said about it, not that what he did to this four year old boy isn't bad enough, but I would like to eventually help him open up more I feel that talking about things will help his behavior tremendously. There is a little girl I babysit who is three years old. Her parents just started potty training, (they waited until she could tell them when she has to go) but when she is sitting on her little potty, or getting a diaper change on days that she doesn't want the potty, his nose is literally right between her legs. The expression on his face and just the way that he stares at her makes me wonder about the possibility of sexual abuse. But I don't know how to approach the topic with him.