Hi K.,
You might also think this is interesting: I am allergic to penicillin. I broke out in a rash all over my body when I was a baby and took it for something. I have never taken it since, but I can take amoxicillin and eurithromicin (I don't know how to spell them), and I'm just fine. However, in my early 20's I noticed that every time I ate beef, I got a little rash on my face, and I thought I was developing an allergic reaction to beef. My doctor said no, that I was reacting to the antibiotics they give to the cows, because of my penicillin allergy. So now we buy beef with no hormones or antibiotics, and I have no more rashes. You may need to do this with your son as well. No big deal--just something to be aware of.
Good luck to you and your little one.