Karen, you will know - at the right time you will know.
Until then honey, remember that there was a reason this creature came into your life to give you the support and love you needed and, I believe, to allow you both to experience the bond between species. When he is fulfilled and knows he has given and recieved what was needed he will let you know. Until then you fight your corner and his - when the time comes it will be hard, but it is OK. All souls here have a reason to be, they sadly have a reason to move on too. One of your reasons to be is to recognise and celebrate that. To have such an exceptional friend means your soul is an exceptional soul that deserves such as him.

Don't worry about hanging on too long - I promise you that you will know when it is time - and don't worry that you are saying goodbye. Life seems such a long time for us humans, it is less than a blink of an eye in the great scheme of things....