Inappropriate behaviour at school

I'm really worried about my 9 year old boy. He's in grade three, and always been a bit of a show off and macho man, trying to impress others. This gets him in strife at school, naturally.
Lately, he's been talking about rude stuff and inappropriate stuff at school. He got in trouble yesterday for swearing (the f word), talking about condoms, dildoes and smoking weed!! I am utterly mortified!! Never once has any of those things been mentioned at our house, and he certainly doesn't watch them on TV.
I'm really concerned about why he'd be choosing to say these things, and to be honest, the other kids at school don't think he's funny, they think he's acting like a jerk (which he is), and tell the teachers when he does it.
I know most people would say it comes from the home, but it honestly does not. I'm pretty sure some of the rude stuff comes from my step son, who is 10, and told my boys he's seen his 18 year old brother and his girlfriend engaging in a lewd act in the older boys caravan. Mr 10 tells me he looks at them a lot. I've told my husband, who has told his former wife, but she doesn't believe him.
This boy is in my sons class, but he doesn't go around saying that stuff, it is my boy who I'd bring naughty!
Please, any ideas why, so I can help him stop it??