Long story short he got my son a phone (its not his son and I was totally not for it) and now that we are no longer together we are sharing the bill...
send the phone back to the aunt. it's really NOT your problem unless you make it so. you didn't sign any contract, you are under no obligation. read that again NO obligation. take out the sim card and be done with it, send it to the aunt or the carrier.
as for him saying what he's going to do....if you weren't still reading texts (on the boy's phone or yours?) you wouldn't even know. again you are under ZERO obligation to respond or react. and telling his family just keeps YOU in the mix. you are reading a whole lot into things....whatever he does, is on him. and if you are so worried about this "just out of prison after 11 YEARS is actively using AND dangerous" then what's up with him being your best friend and all that??? sounds like you need new friends!!!
stay no contact. enforce YOUR side of no contact. get rid of MrApp or whatever. get rid of the phone. be truly done. no more drama.