Doctor or not?

[TheFuckfaceWhisperer]

DS is 19. He also has autism and severe Dyspraxia. He’s never been a great eater but instead of growing out of it and getting better he’s getting even fussier. He often leaves his tea, he has a whole host of excuses why...it didn’t taste nice, it’s not right, he’s not hungry, there’s too much etc. He gets a light dinner every day and something for supper but rarely eats all of his tea most days. His BMI is 20 (6ft 2 and just over 11stone) and whilst he’s not underweight I am worried he’s not eating enough. Would a doctor be interested and could they even do anything?

No, they probably can't. Sorry.
His autism (and associated sensory needs) are the main issue and a doctor can't 'fix' those.
I'm not suggesting any of the following will work but as a for instance:
Go with what he will eat and give him as much of it as is reasonable
Think of what he does eat and build on it, e.g., if he likes beige, extend the beige foods. Chicken nuggets may mean fish fingers are acceptable, pasta could move to noodles. Crackers could become bread sticks.
If he is eating sufficient over a day, try not to worry too much. Does he respond to motivators/bribery at all?

No bribery! All he'll eat is Super Noodles, plain egg and cheese omelette, toast, soft boiled egg and soldiers or cheese pizza. And at 6ft 2 I don't think toast or egg and soldiers are a substantial tea day in day out! A typical day would be:
Super Noodles for dinner
Omelette for tea
Toast for supper. Roughly 800 calories. Plus two bags of crisps over the day. Roughly 1300 calories. Sometimes he doesn't want supper. So minus a few hundred.
He also doesn't like "too much" of anything. The butter on his toast has to be spread thin, he hates the little lumps of butter, the cheese in his omelette has to be a small amount, the liquid in his noodles has to be bloody evaporated! It's just all so stressful, I thought we'd be getting better about this by now

Ignore the 800 calories bit, I was still adding it up in my head and forgot to remove it!

Dr won't do much. What does he drink? Can you fortify those?

Diet cherry Coke or Options hot chocolate! Oh, the odd cup of tea. Occasionally he'll have milk shake (only Frijj ones) but has to be in the mood so I buy them and they usually go out of date before he's in the mood for them! Same with yoghurts. Both often end up in the bin