So sorry to hear that you have to go thru this. Always remember that you and your family gave him 15 wonderful years of happiness. He will always be in your hearts and memories.
He will go join all the other animals that have passed to the other side at the Rainbow Bridge where he will wait to be reunited with you again some day.
I have a 10 year old boxer who has cancer and I treasure every single day I have with her because I just don't know how much longer she will be with me.
My prayers are with ya.