Parent of a kid with peanut and tree nut allergies. Lunch options include:

Cream cheese and jelly sandwich
Avocado sandwich
Butter and jelly sandwich
Cheese sandwich
Turkey and cheese sandwich
Ham sandwich
Apple butter sandwich
Black beans and rice
Kidney beans with rice
Chili with rice
Pasta with meat sauce
French bread with brie slices
Yogurt and fruit

A jelly sandwich, fruit, and a yogurt are all full of sugar. I would find something else to replace the jelly sandwich. How about cheese? Or cheese and turkey?

Think healthy fats OP. Avacado is a great option. Not for lunch maybe as it will go brown but gif dinner or breakfast a couple of times a week. Will ge eat cheese and crackers? Or a cheese stick?

Are they underweight?

Just give him a full fat yogurt and don't worry about it.

Beef jerkey, pepperoni, cheese. I have the same problem. My kids don't do sandwiches and are super skinny. Their pack lunches are cheese, crackers, some type of cured fatty meat, fruit, and sometimes yogurt. I always send a milk, sometimes they drink it. The little one is really into nuts these days, so depending on the rules of the camp/school/activity I send nuts.

Bagel w/cream cheese
Cheese sticks
Use the full fat yogurt to make a shake (w/fruit) Maybe drinking it is easier, more appealing

Cheese, crackers, and pepperoni.

Cream cheese and jelly sandwich? (Gross to an adult but I loved it as a kid.) Or throw a stick of string cheese or Tupperware of cheese cubes in with the fruit/sandwich.

Avocado or guacamole? With chips.

Bagel with cream cheese and plank-smoked salmon, guac cup with tortilla chips, cheese sticks, a creamy salad dressing for his vegetable dip (may promote those going down too and maybe roast them first in an oil, I halve baby carrots and roast until crisp at the edges and send in lunches), beef sticks, laughing cow wedges with pretzel sticks, pigs in a blanket

cream cheese
cheese
ham, bacon, pepperoni, sumner sausage, etc. I know this sounds awful to an adult palate, but as a kid my favorite sandwich was jelly and sliced summer sausage

Don’t worry too much. Let him eat what he’ll eat for lunch and he can get his fats in at dinner.

I’d lean on the pizza. Make it homemade with whole wheat crust and extra cheese.

Can you sneak butter on the jelly sandwich?

My 5YO won't eat any kind of sandwich except for jelly. He typically gets a jelly sandwich on whole wheat bread, a full-fat yogurt, and fruits/vegetables. He rarely eats the vegetables, always eats the sandwich and usually eats some of the fruit. He has not been eating the yogurt lately. He has food allergies including sunflower seeds, eggs, peanuts and all tree nuts so nut butters are out. We tried flavored tahini and it turns out he is allergic to sesame seeds as well. We have sent dinner leftovers in a thermos but he doesn't usually eat much of it, with the exception of cold pizza, which he will eat.

Essentially, he is a picky eater who also has food allergies. His older brother is a foodie who will eat almost anything, and he also doesn't have any food allergies so navigating this is new to us.

Things that he will eat at home include rice, chicken, fish, pasta, chicken nuggets, waffles & sausage. He just doesn't seem to eat those things when we send them in his lunch as leftovers.