I'm missing two molars on the bottom, and had to get two teeth (behind the incisors) pulled from the top when I got braces because my mouth is so small. It was great that I didn't have to worry about getting any pulled on the bottom, thanks to evolution taking care of that for me
I think you should see an orthodontist -- it's possible you'll have to get an implant, but maybe not (depending on your DS's mouth).