I have one of those too. I have tried many different solutions and have no perfect answer, but here are some thoughts. First, explain to him that breakfast is the fuel and energy for his body and brain for the first half of his school day and is important and necessary, in other words, you will eat breakfast and that this is not an option. Second, and equally important, is that you acknowledge his discomfort and let him know that some people simply have more difficulty with eating early in the morning and that this is okay. Ask him what he thinks he could eat and try to work something out that would be mutually agreeable.
Portions can be very small.
Fluids are very important, but water is just fine and easy to tolerate and balance is crucial. For example, a quarter of a peanut butter and jelly sandwich, a small piece of cheese and a handful of raisins or grapes along with a glass of water would be just fine, or perhaps a slice of turkey and a half of a banana. Even a half of a fruited yogurt with a glass of water is better than nothing. I think you can work this out. These are all different medications.