Agree stupid advice! I was given similar by a paed ‘if he lived in Africa, he would have to eat rice’
take it with a pinch of salt and practice your eye rolling skills!! 
Ds has been severely anaemic, b12 and vit D deficiency too. Might be worth getting a blood test. Is your ds underweight?
We saw a dietitian, she was good at analysising ds’s diet and providing a tub of powder (can’t remember what it’s called now) to replace vits etc. Only problem is ds won’t drink it! We have a b12 spray and over the counter chewy vits.
We have also tried various OT’s including specialist feeding clinic at Evelina hospital. And a CAMHS pilot food group. No success.
I’ve given up! I’ve found any kind of pressure around food, makes it worse!
I think for my ds his extremely restrictive diet is definitely sensory. Also very anxiety driven, when anxious, it heightens any sensory difficulties and food doesn’t taste or feel the same. Also very much about control and routine and not liking changes. When ds gets extremely anxious (usually school related) he restrictis his already restricted diet to literally nothing.
Ds ‘regular’ diet is bread, Rice Krispies (without milk) salt and vinegar crisps, chips, certain salad and bacon. Also very specific cakes or sweets. He isn’t underweight, there are lots of carbs. Just lacking in vits etc. He has the same thing for breakfast, lunch, dinner every single day!
Things that have helped him to at least try a new food-
Reducing demands or expectations
Trying to expand with food that is very similar to the ‘beige’ food he does eat
Trying a new food in a different place. For example my ds will only eat certain food at home and certain food in school. The 2 cannot cross! Go to a restaurant and he has tried a couple of new foods! Which I can then use at home!
I take ds shopping, give him a basket and tell him to put in any new food he likes the look of.
I have also explained what food is healthy and why etc