What and how to say goodbye to my brother-in-law in hospice?

Perhaps share a memory. "I'll always remember that time you ... You are an important person in my life, and I'm so glad you are a part of it."

Tell him you love him and what his friendship has meant to you.

There is a beautiful thing you can say:

What do you need right now? What do you need from me right now? I LOVE YOU.

Help him understand that his life has been worthwhile and contributory, to you and your family. He may be thinking of how much he would like to do, to live for, so reassuring him that he's made a significant contribution to your life may help ease the fact that he won't be able to achieve some potential goals he had. But don't say it that way; emphasize how much he's meant to you.

And I'm so sorry to read of his situation; pancreatic cancer is such a devastating cancer.

Listen to the positive advice you receive here; don't listen to the hospice suggestion of telling the patient, "It's alright to let go." I said that to my husband when he was under hospice care. A few days later -- although I had not brought up the subject again he said, subduing anger -- "Don't tell me it's alright to let go."

Just say " I love you and you will always be in my heart."

Just be there and hold him. Show the love, share the love, speak the love.
I told both my parents that I understood they had to go and that I would see them again when my time came. 

Remind him of the great moments you had with him. Tell him you love him

arianne777, I think some people would like to be told they can let go. I would not suggest that as something to say to a BIL, though.

And I think I would phrase it a little differently, like "Mom, I'm so glad you and Dad raised us to be independent adults, able to function well on our own."

Say what’s in your heart, that can never be wrong.
Please don’t be afraid to touch him...in my experience of working with Hospice patients over the years I’ve seen time and time again friends and loved ones feeling unsure of what to do when visiting with a dying individual.
Touch is important in this life and it can be so comforting to someone who may be frightened or feel alone in their journey toward the end of their life.
A simple hand on a shoulder or a gentle arm rub can make all the difference in calming and allowing your loved one to feel a connection.
Certainly make sure where you decide to touch is free from pain as you’d not want to cause any further discomfort.
Bless you for having such a caring heart.