Hi and welcome to DWD.
Sorry to hear that things are difficult for your brother. It's good of you to go looking for help for him.
Until he is ready to accept help there isn't that much you can do practically - I know that sounds defeatist, but he has to want to get better - then GP is the best option.
However you can do things like make him aware of helplines such as Samaritans, CALM, Shout etc He could also go and see a relationship counsellor such as Relate - you don't have to be in a relationship or go as a couple to get their help.
Does he have any hobbies? Is he eating and drinking properly? What about getting some exercise? These things will all help too.