Hi.. Thanks for the query.. As per your complain in case if your son is allergic to Penicillin then he must still be allergic to it as antibodies once formed inside body against a specific allergen, remains within the system and as soon as the allergen enters the body the antibodies try to remove them.. So avoiding Penicillin group of antibiotics is a good idea.. As far as Keflex is concerned that contains active ingredient Cephalexin, it is a Cephalosporin group of antibiotic and in case if he is allergic to penicillin, it is not necessary that he will be allergic to Cephelexin too and he can very much take it.. If at all you find any allergy symptoms after taking it, you should not take further doses and consult your Physician.. Along with it he should do warm saline gargles, suck throat lozenges, do steam inhalation, chew a vitamin C pellet and take a soft diet... Avoid spicy and chilled foods and drink plenty of warm water.. Hope this information helps.. Regards.