Just be there. That’s all. Do not force a conversation – he will smell it out right away and shut down. Underneath it all he knows its harmful but he is hurting very badly and his addictive tendancies will be with him for life. He needs your support when HE is ready to open up. If you can learn his patterns you can try to engage him in other activities during his worst times – even out of the Beis Medrash eg the handball court or track etc. You can approach a rav or rosh yeshiva who can be in touch with him and/or his family – a rav is not a “friend” who is badgering but rather a teacher/mentor and it will be taken differently.
And of course, Tefillah is the key to everything. Add his name to your tehillim list – but not the mishaberach list in shul!