My daughter has life threatening allergies to dairy, amoxicillin and cefprozil and at one time allergies to eggs and soy. She has worn a medic-alert bracelet since she was 2 and they will size it smaller if the sizes available are too big for your son. She had no problem wearing it and likes that she can choose the color.
Also, when your son starts attending preschool, I would recommend purchasing a product called SafetySack. It's a heavy duty plastic ziplock type bag that has a squeeze lock on it so other children can't open it. It's clear which is great, because everyone can see the instructions that I have for the order of medications to administer. Plus it is big enough to carry two epi-pen jrs, the epi-pen tester, benadryl, prednisone, and a dropper. We have two sets, one for home/playdates and one for school. I highly recommend it.