Is my DC moonlighting as your DC?! They clearly got the same eating memo. He won’t touch anything in a sauce at all. To the extent that I’ve stopped trying.
He will eat pasta plain(always)/tossed in butter and maybe a bit of Parmesan (Sometimes). So if we have spaghetti bolognese, he has spaghetti with a side of fish fingers (think I prefer fishmongers 
).
He has branched out since starting school and will eat the following in addition to your list:
- carrot (raw only)
- broccoli
- peas
- sweetcorn
- roast chicken
- plain grilled chicken (I brush with lemon and olive oil first)
- roast potatoes
- sausages
- burgers
- hot dogs
- grilled/baked white fish or salmon
- battered fish
- Quorn nuggets
- jacket potatoes (including with cheese and baked beans!)
- mashed potato
- ham sandwiches
- cheese sandwiches (he’s gone off these)
- gammon (as in roast joint and sliced rather than grilled gammon steaks) - he’s not mad keen but your DS might like it?
If I had more time and it wasn’t so disheartening, I’d try home made fish cakes and nuggets, basically versions of the processed stuff he likes.
To be fair, he ate some of the above before he started school but he has branched out.
I have just focused on trying to increase variety and accepting any success, even limited, as a win - while reminding him that we can’t always have our favourites. Lots of praise for trying new things, absolute indifference to whether it’s eaten or finished.
My main goal has been to increase the quality of his protein intake and reduce the level of processed foods in his diet. So grilled/baked plain protein, for example.
We have had issues with food since he was 9 months old so it is basically how he is, if you see what I mean, and I decided early on I wasn’t going to get into battles over food, especially as he is on the thin side for his age.
I am not saying my approach is the right one, and I have concerns around his diet, but it has made mealtimes less confrontational than they might be and he eats a wider range than at 4.