I presume you mean you are taking him to the vet tomorrow?
If you are then the vet may suggest some medication or tests that will improve his life.
If it comes to the worst then only you can make that decision (you are the only one who truly knows the dog), the vet will advise you and I would really listen to what they say.
Be totally straight with the vet, tell them what you are thinking and ask what they would do in your situation.
If it comes to the worst you can either leave the dog with the vet or you can stay with them until the end (I always stay with mine, but my OH cannot stand to be there). You can also call the vet out to your house if you wish.
Decide beforehand what you want to do afterwards. Some people bury the dog in a 'special' place in their garden. You can get them cremated and their ashes returned in a casket (usually about 100 pounds). It may help to write down your wishes - you may be too upset at the vets to talk.
It is hard when they go, you never 'get over' it, but after a while you do remember the good times and the pain does subside.
Sadly it is something all pet owners have to go through - we don't think at the time we will get through it, but we do.
Be strong for his sake, he really needs you now more than ever before.