What to do about Duke.

Duke, our old new england that we rescued from a fighting ring several years ago, got kicked by the donk and got himself mortally wounded. Had him to the vet who several days ago said it appears as if his chest bone was broken but said as long as he is up and around and eating well, he might be ok, as long as there was no internal bleeding. He did alright for a few days, but this morning I found him on his side, not moving. No reaction when I called him and cold to the touch, though he did raise his little head when I petted him. No doubt in my mind that he is dying. Vet says best way to end his suffering is to "give him the axe." God I hate this part of loving them. Thinking now what shall I do? I would really rather he go on his own but how long should I wait? What would you do? Advice please..

Update: Took Duke to vet same day to have him properly euthanized. RIP Duke. You were a good boy.