Please help me find something for this very generous man...?

A nice thank you note and your love should be enough. Make him proud of both of you and if and when you have a child maybe you could use one of his names for the baby's name or a name of someone who was very special to him. We gave our son my grandfathers first name as his middle which was also my fathers middle name. We found out later that it was also a name from my mother-in-laws side.