allergy

My baby has been fed breastmilk exclusively for the last 5 months. My supply is going down and I have decided to start introducing some formula. We did one oz on S and M, 2 oz on T and w and then 3 oz on Thursday. Well about 5 min after the bottle on Thursday my baby was puking, covered in hives, sneezing and eyes were swelling shut. We went to the ER and they gave him shots of Benadryl and steroids. They are thinking he might have an allergy to milk protein. He will be allergy tested on June 20. I was given Alimentum to try that but now I am so scared to give him new things! Does anyone have any experience with allergies and this formula? Did it work? Also, doesn't it seem like waiting until June 20th for allergy testing is too long? Thanks!