J.F.
First of all I think it is awesome that you support him. I have a 2 year old who has a gay grandfather who is married to a man. It is Grandpa and Shane to her and she is young enough that she doesnt really question it but when she does I will just explain it in as simple of terms that I can that is age appropriate. I think telling her that he will marry a boy instead of a girl is definitely as much as she would need to know at her age. I dont have any books to recommend, but I would love to find one too! Maybe you should write a childrens book about it! :)
I think a small get together/ dinner/ cook out at someones house would be cool. Maybe have a rainbow cake?
Anyway I hope you dont get any negative responses. I think its great to see other open minded accepting people in this world!