I want to try wool for my DS for overnight. He is 4 months old, but he is a big guy-- 28 inches and 18+ pounds. I want a good fit, but I don't want to get a size he will wear for only a few weeks. I am looking at Disana wool covers, and the medium starts at 20 pounds. (The small goes up to 21). I have no idea how wool fits, so I am not sure what I should go for. Thoughts? Insights?