unless you were there at the GP consult you dont know what your brother said nor what was said back...
Gp can only act on what the patient says...
when i atended an appt with my (now ex)P he was insistent he just had problems sleeping and needed some help with that - despite GP going thru depression questionnaire with him and saying "you are depressed" "you need help".
he is an adult - unless/until he gets "bad enough" to be sectioned, only he can seek the right help, counselling, whatever is needed.
the GP persuaded my exP to try extremely low dose antidepressants - as a way to get him to take them... he was very insistent he didnt want/need them, he was just "tired" and needed more sleeep etc etc... (he didnt take the anti-Ds, got worse and worse over the ensuing weeks til yes "exploded"...)
what does his wife say?
can she provide evidence of his behaviour etc to GP in an appt she books for herself?
how is it impacting on her/any children?
one male acquaintance has told me how it was his wife telling him "you get the right help and take medication or i am leaving you" that was the kick he needed to see what was going on and do something....
perhaps you can be the one providing him with the support he needs (eg could he come stay with you??) while his wife has to be the one issuing ultimatums?
you could find out about free or reduced rate counselling available locally...ask about GP prescribed exercise programmes in your area that kind of thing and pass it on...but ultimately only he can take the steps...