What Causes Nervousness And Tremors?

On May 7th, 2010 my boyfriend had an extreme allergic reaction which forced him to go to the hospital. Before going, his dad gave him a pill which he was unaware was Zotex. At the hopital he was treated with four shots, one being benedryl and the 3 others to treat the reaction and cure the itchyness. However, shortly after going home he said that the medicine was much too strong and began to expierence extreme nervousness. It wasn't until he ate something that he got better. As the days progressed from there he suddenly gets this nervous feeling out of no where, and with each day the attacks have become more and more frequent. At first I figured he developed a panic attack disorder, but then, two nights ago he began to also have aniexty attacks in which he would all of a sudden start crying and think someone was in the room/in his closet to kill him. Eventually it got worse and he began seeing images in his head of dead people all around him and trying to kill him. These really bad attacks usually occur during the night. At the same time he'll begin to tremor and have something that looks like it could progress to be a seizure. His heart beats uncontrollably until I settle him down into deep controlled breaths. As well his body sometimes tingles. He's scared he's going to die but doesn't want to consult his parents because he doesn't want something to be wrong with him. is this a really bad side effect from the Zotex (he only had it once on last thursday night) or is this a major problem shaping in his health? please help