I appreciate the input, Justice2, but my sons symptoms differ in a few ways. (They did, rather; he's starting to get better now, thank goodness!)
1. Measles causes characteristic spots in the mouth, on the insides of the cheeks. He had these.
2. His rash started behind his ears. This is a very strange place for an allergic rash to begin (unless it's to earrings or shampoo), but it's usually where the measles rash begins. It's spreading pattern was also very suggestive of measles.
3. His rash doesn't itch. Allergic rashes nearly always itch fiercely (and I've had several of different kinds, so I would know!), but the measles rash usually doesn't itch at all; it just looks like it should.
4. His eyes were swollen and he screamed bloody murder when exposed to light. (Again, very common with the measles, very rare with allergies.)
5. He ran a very high fever (104.2 was the highest I temp I recorded, and I'm willing to bet it was higher at some points

). Some people are inclined to run fevers with allergic reactions (I am one of them), but they are usually much lower, even in infants.
6. He's still got the rash. Allergic rashes to medication generally disappear within 36 hours of the last dose; Eli last had Amoxil at 8 am on Monday.
My mother is allergic to penicillin, so I have no problem with listing an Amoxicillin allergy in his chart as it is still a possibility. While measles is very rare in this country, in areas of the country with large unvaccinated populations (for example, places with large Amish communities) it is more common, and I happen to live in one of those areas and visit another weekly (south & central PA).
As a result of my research and the opinions of others (including his pediatrician), I have been taking precautions based on the idea that my son had a contagious disease, which I thought was the more polite thing to do as we are, at least weekly, in contact with children too young to have been vaccinated and potentially too young to deal well with the disease. I made it a point to inform people with very young children with whom Eli was in contact this week, as well as pregnant women, that they may have been exposed to the measles. Better safe than sorry.
edited to fix smilies. 
Follow Mothering