Mine are also fussy eaters.
Can you try to find bits he likes and make them into different meals. Mine also like burger and chips and so will eat eat meatballs in a basic tomato Sause (onion, garlic, oregano, tinned tomato's) with pasta. As meatballs are similar to burgers. The pasta sauce is a similar taste profile to ketchup and pasta is as bland as chips. Could you try that?
From there you might be able to get a bolognaise (mince is burger, your basic tomato Sause just has added flavors - try just one ingredient at a time for a gradual change from tomato to bolognaise)
Does he like baked beans - mince and beans is the start of chili con carne - could you try rice?
Mine don't like (mixed in) foods, like it separate. They will not eat meaty meat (cant seem to get on with chewing)
But they like hummus so they have it with most meals with cucumber, carrot and red pepper sticks to get some veg into them.
Lots of meals start with a carb (mine love bread) rice or pasta (they are not keen on potato other than fries) Then a protein. They will tolerate fish fingers, breaded chicken (nuggets) and sliced ham as well as mince) Then a selection of bits they will eat, sliced apple or grapes, veg sticks and some kind of dip - ketchup, hummus, peanut butter. The more random selections of food I put on a plate the more they ate as I popped mystery items previously untried or unliked which were tolerated. Then I added some of these new acceptable foods together to build meals.
Its been hard work, time consuming and frustrating but I can see that they are getting wider diets now.
Good luck.