Brain tumors ? help im confused?

so my boyfriend and best friend in the entire world has a brain tumor and the doctors told him he has a bout a month to live he cant get surgery in canada for it because the surgery he needs is only available if your over 18 and were 14 and his mom was thinking about taking him to the states but im not sure she can whats going to happen to him like not if he dies but how will he die ? because it keeps on growing and hes had a siezure not too long ago and always has these terrible headaches but like how would the brain tumor kill him exactly other than the obvious that its a brain tumor . like thi is hard to explain but i hope someone knows what i mean ? like will it trigger like a stroke or heart attck or just simply do something else