1) you and your Husband are not comfortable with this. Fine. That is your feelings/vibes/thoughts on it. And... this is YOUR child. So, if you/Husband are not comfortable with this, then stop the sleep overs. Or have it at YOUR place.
2) Do you, talk with your son... about your own expectations and head's up with him? ie: do you tell him and does he KNOW... himself well enough that if "weird" things goes on at someone else's house.. that HE CAN SPEAK UP too, and say "I HAVE TO CALL MY MOM FIRST, AND ASK IF I CAN..."? My kids, do that. They are 6 and 10. IF something, feels "weird" to them, or if they KNOW that me or my Husband would not approve or feel "safe" about something... they TELL their friend's parent or even their relatives or whomever they are with. AND they call, us. First. They speak up.
3) How old is your son?
4) BEFORE my kids go over to anyone's house, I ALWAYS tell them to call me... ANYTIME... to ask or tell me anything, that they feel is important or if they are uncomfortable etc. And they do. AND I ALSO, tell the hosting Parent.... "CALL me, if any plans change... or if my son/daughter feels they need to call me for anything, or if the visit is going on longer for any reason." I TELL THE hosting parent... this... clearly.
4) IF you do not "trust" this household/parent/parents for any reason... then that is reason, to not let your son go over there. A parent's "radar" is as such.
5) Then, you said you don't know what went on over there. Did you ask your son???? I always ask my kids, when they come home "what'd you guys do?" "Anything inappropriate?" "Was there a parent there, supervising you all?" Even my 6 year old son, will say something and articulate what they did etc.
BUT also, my kids go to friend's homes... in which I KNOW the Mom, WELL. AND, with my daughter who is the oldest one and who has gone on sleepovers, she calls me WHILE at her friend's house, just to "check in."
6) IF while at a friend's house, and the parents go somewhere else... with the kids, THEY call me. First. That is my rule for my kids and that is what I tell the Hosting parent, to do. As a head's up.
IF while at my house for a play date or sleepover... and we decide to take the kids to the mall or to a park.... I ALWAYS, call the parents FIRST, to ask them if it is okay. If not, we do not go.
7) You do not seem to really know, your son's friend's parents very well, nor are your comfortable with them. And that's fine. But you need to know, your feelings on it and if you want your son to continue going over there or not. And say so. And EXPLAIN to your son. I do that with my kids, even at their ages. They understand.
8) The Knife thing: I don't care if this is with boys or girls or what age they are... I would NOT think kindly of that. Why the hell, did the Dad need to show your son a knife anyway... and tell him that he can feel it AND that HE CAN USE IT TO DEFEND himself, against invaders??? That is weird. That isn't even his own kid. I wouldn't do that with any of my kids' friends that I was hosting.
9) Then, the Dad left your son and his friend alone to pick up his wife from work. He was gone for 20 minutes. A lot can happen in 20 minutes, REGARDLESS of age. It doesn't matter that your son thinks he is old enough to be left alone. YOU are the parent. And this is another home. Not your own. And the Dad of that friend... does NOT have the right to decide this... this is not his son. The DAD... SHOULD HAVE CALLED YOU FIRST.... to ask you. AND being he had another person's child in his home as a "guest"... he should not have left another person's child... alone while he left the house. IF I have a guest child in my home... there is NO WAY, I would leave the house, leaving my kid and another person's kid, home alone. No way.
10) then, the Mom took your son and her son to another person's house, without telling you first, and without calling you first and asking you first. That is to me, not okay. How long were they even over there? Why did they go? What did they do over there? You don't know, right?
Okay, so all of this is my own opinion and what I do and what I do with my kids. And this is my take on it, per your situation.
Too many times, a parent feels hesitant to say anything... for fear of upsetting the other parent. But, this is YOUR child and you need to decide, which is the priority? Pleasing that other parent, or looking out for your child?
The thing is, you do not know those other parents nor their family/house very well. And you do not trust, them.
Do you even know that other boy, very well????
What kind of kid, is he?