have a read about valerian root for cannabis withdrawal also u can go to see a herbalist who will tailor doses to him
look up Narcotics anonymous meetings near him or call them for advice, u could maybe attend a few with him for support
calk drug service and tell them he’s suicidal, they may put him higher up list, make sure he stays on the list anyway as it’s only a few weeks and they will give good advice and support and have counsellors there
maybe see if he would stay with family for a while if it’s feasible
good luck I really feel for you all.