WOW, timing is everything. How many people will be in attendance? It sounds to me like he is not into a big flashy wedding but rather a lovely ceremony among his adopted family. As long as your house is clean and you can accommodate everyone to sit, you'll be fine.
Whom can you enlist to help with flowers, setup, and deco?
What kind of food will you offer?
Just think of it as an impromptu party and all will be well.