On the one hand, I hate onions. So I totally get it. I wish I could sue every time someone sticks one in my food as a disgusting surprise too.
On the other hand, if you have a food allergy, that is different than just requesting "no onion" on your burger. They have to take steps to prevent cross contamination. It is a whole thing, and he should know that if he is really that allergic. He would be having this issue all the time becuase (as I well know and lament as an onion hater) onions are in a ton of foods everywhere you go.
They would only be negligent and liable if he told them that he was allergic and they claimed to have taken precautions to prevent exposure of his food. If he just asked for no onions and had an allergic reaction because they messed up his order like every fast food restaurant in the world does sometimes, that is not gross negligence, that is a standard accident.