By using my super-sleuth skills, I finally realized that the problem was either with my milk or my freezer. So I took the websites' advice and scalded a few ounces of milk and then put it in the freezer for two weeks. Then I took a regular bag of milk and stored it in my babysitter's freezer for two weeks. It ended up that the scalded milk was fine and the regular milk was bad. So my freezer theory was wrong. I'll admit it in public, "my husband was right."
Although it is somewhat inconvenient to scald all my milk before storing it, it's still worth the effort. It will be interesting to see if this problem returns with subsequent children. Also, from what I've read so far, scalded milk lacks some of the nutrients found in fresh milk, but since Jack only gets about one or two bottles a week I'm not too concerned.
So thankfully, the mystery has been solved. Now look how happy this boy is!