My daughter is allergic to cephalosporins as well. She did not have the trouble that you describe at all, and she did steroids as well.
I would say have him checked again. As you may know, sometimes you can develop cross-allergies once a reaction takes place. I know for me, for example, I became allergic to neosporin, and once that happened, I developed a cross-allergy to many other things. I wonder if this could be the case with your son. Perhaps he just became super-sensitive to something that wouldn't have otherwise bothered him during this reaction period.
Fun times, right? :-) Good luck...