I was out yesterday with my ten-year-old; he seems to be of the same opinion, .22lr to the neck.
I have used 12 ga., .410, and .22lr, but yesterday I used a 16 ga. side-by-side for the first time and have to say its a lot of fun. I suppose I base my choice on how I feel on a particular day. Today, I think I'd go with the 16 ga. If I think I may have a chance at surprising some ducks, or maybe a goose, I'll take a 12 ga.. If either of my sons are with me and I want them to do most of the shooting, it'll be either the .22lr or the .410 or both. If I'm looking to brush-up my skills for larger rifles, I take the .22lr. They're all fun.
Ammo for the 16 ga. can be hard to find, but its usually pretty inexpensive and I buy enough for the year when I come accross a supply. I agree that the .410 is getting a tad expensive.