He has been on the antibiotic after skin testing and an oral challenge probably 3-4 times now since the oral challenge.... should we still withold that antibiotic or consider it safe since there have been no problems on it at this point?
If there was a temporal relationship between the antibiotic administration during infancy and the development of Erythema Multiforme (EM), it is highly likely that the EM was caused by the antibiotic. In that case, your son should never be given that antibiotic or any medicine in its class, again. If it happens to be a one-of-a-kind antibiotic for which there is no good substitute, one he could conceivably need in the future, you would do well to consult with an infectious disease specialist now, rather than waiting until such time, as the need for it is great. The allergist's observation and conclusion may be correct but you should not put your son at risk of having another severe reaction, on the basis of one negative allergy test.