If he's not in pain and is not suffering, treasure the moments you have left with him and let him live out what time he has left with you. Trust me, you will miss him when he's gone. Pamper him, let him sleep and give him what love you can without ignoring the others.

He may be old and slow, but if he's not in any pain or suffering, why end his life now? Seems senseless to me. If you don't have the desire to keep him going, perhaps find someone who will let him live out his last months or years in peace.