LOL  GOOD LUCK!
I am 22, buy all the feed, all he has to do is go pick it up and take care of the birds when I am away, and I tried to talk him into them...it was a no go.
So, being the brat I am, I got some eggs, and told him the plan was to give them to a friend from work.  Until they hatched and he decided he liked them.  Now, on occasion, I find him back there talking to them, he makes sure their water is clean when he is off, and is planning for a small pond for them...
Tell him the eggs are big enough to feed 2 people, or that fertile goose eggs sell for more than some chicken eggs. Or just walk around blabbing about geese all day long..do your research and show him that you know how to take care of them. Start taking care of the ducks by yourself (if you don't already) to show you are willing to take care of the geese on your own.
Be an adult about it, if he says no multiple times, drop it for a bit and bring it back up later.