That is a very difficult position to be in, so welcome and good for you for posting. First and foremost: his relapse is NOT your fault. If he is choosing to hang around with the same people who use with him, then point blank he is not serious about recovery. People, places, and things are among the first things we need to change, and if he doesn’t want to commit to that, then relapse is very likely.
I know a little bit about those sober houses, and they can be great; they can also be essentially houses full of bitter, “dry drunks” (alcoholics who aren’t drinking but are not truly sober or in recovery).
You can’t police him or guilt him into becoming sober; he has to want it for himself first. You can be supportive by listening when he wants to share and not being judgmental when he struggles (it doesn’t sound like you are judgmental at all though), but other than that he has to work his program among those who have been through this and know how to relate on a different level. He’s an adult and is going to do what he wants to do at the end of the day.
Try going to Alanon meetings! Great support from people who have been where you are and find hope where there was otherwise helplessness. I hope he finds a plan that works for him, and you find some relief :-)