I would guess that he's vomited or recently seen someone else vomit recently. At 5 he's interested in everything that comes to his attention. This interest will pass if you don't make a big thing of it. Listen to him, answer his questions and be casual about it. He will figure out whatever he's needing to know and he'll stop talking about it.
I don't think vomiting is horrible. It is unpleasant. It is also good when we vomit because it is our body's way of getting rid of whatever is making us sick. Perhaps if you could let him know the good part about vomiting and that it is OK to vomit he would be less concerned.
I wonder if he's associated a negative reaction that he's experienced with vomiting and he's worried about the negative reaction and not so much the actual vomit. Tell him you know vomiting is unpleasant and it can be upsetting but overall it's good to vomit when he's sick.
Ask him how he's feeling when he feels like he's going to vomit. Anxiety can cause us to feel like we're going to vomit. Actually, anxiety can cause us to vomit. Perhaps it would help if you talked with him about things he might be worried about. Perhaps find a book at the library about vomiting and being sick as well as about being worried.
Talk with him about ways to stop worrying. Give him some coping skills for when he is worried. Talking about what one is worried about is one skill. Doing something about the worry is another. For example if he's worried that he's going to vomit, getting involved with something energetic and fun may help.
I suggest that he might have GERD and is actually having symptoms. Perhaps a trip to the doctor would help.