I would suggest “custom spinach pesto”, where you serve a base of plain spinach pesto made from lightly cooked spinach, onion, garlic, salt, pepper, and lots of olive oil. Blended, then mixed with spiral pasta in a very large bowl.

Then, you provide your guests with a very large selection of mixins: halved cherry tomatoes, bacon, fried mushrooms, steamed broccoli, grilled zucchini, olives, capers, Parmesan, grilled chicken breast, rucola, chili flakes or oil, pine nuts, pumpkin seeds, anchovy paste, sun dried tomatoes, baked tofu, etc etc.

I’ve made this numerous times for a crowd that was a mix of allergies, vegans, picky eaters. The spinach pesto base can be either mixed into pasta initially or also placed on the side. The most picky of guests eat basically plain noodles and the others are experimenting with different combinations with each serving.

Serve with a very simple classic salad (lettuce, tomato, cucumber) with a bright acidic dressing, which complements the pasta in both flavor and simplicity.

You may optionally offer whole pieces of grilled chicken or something on the side to make it feel like a more complete meal.

Another simple dish we made numerous times for this crowd was Levantine mujaddara, which is lentils, rice, and caramelized onions with warm spices. It’s a superb dish for a vegetarian crowd, but your picky eater might find lentils and cinnamon off putting. I serve this often with baked chicken on the side. To match the Levantine theme, just bake chicken thighs kind of plain and after pulling from oven coat in olive oil, lemon juice, salt, fresh garlic.

Baked potato bar with options for individuals to add their own toppings (cheese, sour cream, bacon, butter, chives, garlic powder, salt/pepper, etc.) Just pre-cook the potatoes and have ingredients on the side so everyone can customize their own. That should at least cover the starch/side option.

You could do an Italian red sauce, then serve Eggplant Parmesan and Chicken Parmesan. It should be the same prep for both dishes.

Spaghetti and vegetarian marinara sauce, with a side dish of meatballs cooked in sauce. Vegetable side dish with very light seasoning.

Baked ziti is a pretty simple dish, and is boring enough for the picky eater. Just don’t find a recipe that uses meat.

I’ve also done French toast casserole. https://www.foodtasticmom.com/make-ahead-french-toast-casserole/ with tater tots. This is assuming your group accepts breakfast for dinner.

Taco bar works well, and can fit most appetites.

Also steamed buns/dim sum can be grand if you have the steamer baskets.

Could you try and split up some things you normally cook into parts then add the flavor part to everyone but his at the end?

For example roast some chicken thighs in the oven, salt pepper basic seasoning. Saute some veggies and tofu (for the vegetarian) in a pan, again with salt, pepper, maybe garlic etc. Server over rice. Then give him just that, but then you add some sorta sauce to bring it all together? Maybe a curry?

If you’re used to cooking asian foods, you could try something like curry rice with hamburger steak for the meat eaters and croquettes for the vegetarians. It’s not at all spicy (actually, it’s often pretty sweet).

You could also do some kind of donburi like soboro-don (ground pork and egg), and tempeh for the vegetarians.