Omnicef and severe sinus infection

My 16 yr old son was prescribed this drug 9 days ago for a severe sinus infection. He came home from school yesterday with several small raised welts on the creases of his arms. I took him immediately back to his Dr who said he was having an allergic reaction, prescribed steroids and said it would get worse before it got better. Within a few hours his elbows had visible knots in different places, then his knees did the same thing, then his feet started getting knots on the bottom and he had different whelps breaking out on his torso and lower stomach. We immediately took him to the ER last night when he said his tongue "felt funny". The attending immediately diagnosed an acute severe allergic reaction and gave him a huge steroid and Benadryl dose. He released him because the itching had stopped and his tongue no longer felt funny. Last night he kept drenching himself with sweat during sleep. Today so far some of the whelps are gone but he has some pretty severe joint involvement with visible knots around the elbow and knees and feet.