13 year old won't eat

My 13 year old son has been having issues with eating and he's now considered underweight. He was on this medication 8 months ago and it caused him to have a seizure and he threw up during the seizure. It really scared him and he doesn't want to throw up. He's never liked throw up. I mean no one likes up but he's not to person to wish they could just throw up if they're sick. He rather be nauseous 24/7 than throw up. Ever since that seizure it's gotten worse and he's so worried about throwing up that he doesn't want to eat in fear of throwing up. I can get him to eat tiny portions at times but it's not enough. Even his school is worried. They've said he won't eat breakfast or lunch. I'm really worried and not sure what to do. My husband is the disciplinary figure so he's taken a more "aggressive" approach when it comes to getting him to eat but it's not working.