Thanks for all of the responses!
A few clarifications. The meal was not "cheese heavy", in my opinion. Yes, there was SOME cheese in 4 out of the 7 things I made. There were also some form of tomatoes in 3 of the dishes, garlic in 4 of them, thyme in 3 of them, etc. The dishes contained cheese as an ingredient, they weren't JUST cheese or even MOSTLY cheese. I say this because people keep saying things like "I wouldn't expect all of the dishes to be so full of cheese". 3 out of the 7 dishes had no cheese at all, and the others had some cheese, among many other ingredients. Cheese is a common enough ingredient that I personally don't find it odd for it to be in 4 out of 7 dishes, especially for an Italian meal. However, I am truly posting to learn, so I will keep that in mind for future events. The problem is, I still don't know how I'm supposed to plan if I don't know the offending ingredient. What if it wasn't cheese but instead was garlic or tomatoes or something else? Those are all very common ingredients. If I make 7 things and can't have any repeat of a very common ingredient, even in smallish amounts, that gets pretty complicated. Any thoughts?
As far as making something else, truly, I could not have done any of the suggestions listed in the time that I had. I was already serving appetizers and drinks when he said it, and the entree was in the oven. I was extremely busy already bustling around to make sure the guests had a drink, etc. I did not have any other meat defrosted, and I don't keep bottled spaghetti sauce in my house. As someone else mentioned, I avoided doing the regular grocery shopping until after the party, because I needed room in my fridge for the large amounts of party food and drinks. I literally could have offered him maybe a sandwich, with lunch meat only since he wouldn't want cheese. Or a peanut butter sandwich. If I were him, I'd rather eat the appetizer, salad, and bread than that. But maybe I should have offered. Or opened my pantry and fridge and let him look to see if there was anything he could work with. I do think my immediate response could have been better, I was just thrown off. It was frustrating, because all I could think was why on earth didn't he tell me when I asked? Next time if this happens I will try to put that feeling aside long enough to ask if I can get him something else. I do agree that I'm not obligated to do so, but I wouldn't have minded making the offer if I had thought to do so in the moment.
A couple of people made a good point that maybe he was telling me in order to explain why he wouldn't be eating much. That would make sense to me if he didn't go on to eat almost everything offered. No polite portions either, he ate a full meal. I would never expect someone to eat something that disagreed with them and would never be offended if they didn't. No one would have given him any grief for not eating much, but he ate it anyway. So I'm not sure that explanation makes total sense to me. It does however give him the benefit of the doubt, so it's worth considering.
I do think, after thinking about it a few days, that he had one chance to say something and should not have spoken up at the party if he didn't want to speak up earlier. I can not imagine doing something like that at a party, and if I was asked why I wasn't eating, I would take the blame and say "it's my fault, I didn't mention to you that I couldn't eat cheese. You did ask. But I'll get plenty to eat with this salad and bread, don't worry about me!".
I do appreciate the perspectives and will try to vary my menu in the future if possible, however I'm torn because most of the people really did like the meal and it got a lot of compliments. In my experience, cheese is a popular ingredient among many. A lot to think about.