Hi @Caroline277 I'm afraid that I don't know of any way to see what amenities are included in the original booking. Airbnb will have a record of when you made the change but I'm not sure that it will be very helpful. The guest sounds like they are going for their full value, and it may be easiest to just give in for this guest. I doubt many will ask you for this so hopefully it will be an isolated request.
Maybe you can say to them something like "breakfast was not included as an amenity when you booked and your rate reflected this, however I'd be happy to provide you with breakfast for the first three days (for example) of your stay" or else you can tell him that his rate didn't include it but if he wants to pay the difference you'd be happy to provide it.....
Good luck!