I am so sorry you are going through this. As everyone else said, only you & ur mom can make this decision. Also as AnD said, you know him and will know when its his time.
I understand what it is like to have a pet that means the world to you. I have a cat (15yrs old) that I love to pieces. She got really sick (not eating or drinking, loosing weight, depressed, withdrawn...totally opposite of what she normally is) a while back (due to what we found out was a slightly enlarged heart). I was so afraid I was going to loose her. I sat by that cat like a mama with her baby. I knew in my heart that couldnt give up on her. DH and mom told me to be ready for the worse. But I just knew it wasnt her time. I did pray for God's will to be done (I know, prayers for a pet seems silly, but in my eyes she is my child), show me what to do and I will except the outcome. I took care of her, gave her tons of love and affection (in case goodbye was the answer I would have no regrets) and I waited to see what would happen. Fortunately, in this case, she got better.
You know your pet, listen to your heart, pray about it if you need to, and you will know what to do.
Good luck. Please keep us updated. (((Hugs)))