B.B.
We have had to put down a black lab that had a tumor on her nose. It is sad to have to make that decision. BUT you say he's 14 YEARS old thats like 98 plus years in dog years. So I would say he's had a long loved life. IF he does have tumors then as it progresses his life will be greatly deminished.
My advice would be to get a puppy now. Then IF/ WHEN he is put down or goes naturally your children won't feel such a loss. And having a younger dog helps the older one... (don't know why, but it seemed true for all of our dogs. ) The older ones teach the younger ones respect and make training them alot easier. (showing them an example and the puppy tends to just follow it) Also the kids will play with the younger and give the older some alone time to rest if he needs it... But he'll still get involved when he wants to...
I only speak from the way we always did it. It is truly a personal choice. No matter what you chose, I know you truly have the well being of your loved pet in your heart.