So this is a little confusing and it would take me hours to explain the entire backstory, so I'll do my best to sum this up.
BM has three kids, all with different dad's. Here's how I'll break it down to somewhat simplify. BM, DH, and SS are easy.

K1 = BM's oldest
D1 = K1's dad
SM1 = K1's stepmom
Doofus = BM's husband

Here it goes...
This weekend DH got a call from D1 asking if SS had mentioned anything about Doofus being too aggressive with discipline. They talked for about an hour and after DH told me the following:

When BM and Doofus went on a vacation with D1 and SM1, they had all been drinking when Doofus told SM1 that he sometimes has to slam K1 against the wall when disciplining him, because he doesn't listen. K1 had never mentioned this to D1 or SM1, but since it was coming directly from Doofus they believed it to be true. D1 talked to K1 about it once they were home and he confirmed that it does happen but it never hurts him. D1 also mentioned that K1 told him, Doofus one time shoved SS while he was trying to go upstairs to say goodnight to BM and he fell and hit his head. D1 did admit that K1 told him about this over a year ago and couldn't remember exactly when it happened.

DH and I thought back and don't recall ever seeing any unexplained bruises or cuts on SS and he's only mentioned Doofus putting his hands on him once (spanking him - 2 years ago) and DH addressed it immediately with BM and since then SS hasn't mentioned anything.

So - DH obviously wants to talk to SS about this, but wants to be very careful how he approaches it...If there is something going on there - too aggressive/abuse, etc and it's something that leads to police involvement...DH wants to make sure that the information is received correctly, according to the law. So he obviously doesn't want to say something direct like, "is Doofus hitting you?". Any suggestions on way to address it, without directly calling out Doofus/"putting ideas" into SS's head? SS does occasionally see a therapist for behavior issues at BM's house and he has never mentioned anything like that, but I don't think the topic of Doofus is brought up too much. It's typically focused on why he lashes out at BM, etc. CPS has also done two home visits in the last year (they had a rapist living in their basement - a friend of Doofus - much longer, more frustrating story) and both times they found it to be a safe environment, and therefore DH was stuck with his hands tied. Luckily that is over now and the guy has moved out.

While we are going to take this as a serious thing, I'm cautious about information received from SM1 and D1. First and foremost I'm annoyed that D1 wouldn't have told DH about the stairs incident immediately, instead of waiting over a year and DH didn't get the impression that they are doing anything to stop Doofus from slamming K1 against the wall. Also, D1 and SM1 are on again/off again friends with BM and Doofus and typically when they've had a falling out with BM, they suddenly want to be our friends. I don't think they would lie about something like this. We actually like them and we get together occasionally with them so all the kids can spend time together (K1 and SS don't have the exact same schedules so they are only at BM's house at the same time, 1 day a week and every other weekend) so D1 and DH do their best to make sure they get more time than that together (both dad's have offered to adjust their schedules to match the others, so the boys have more time together and BM said no - the reason why is beyond me). Anyways, I don't think they would make it up, but I always worry about what they tell us, and typically we just listen to their ranting about BM and just put all the info in our back pocket because we aren't interested in getting involved in all of that drama....but this information is clearly something that we cannot "save for a rainy day".

Any suggestions?