lollardfish

The eating therapist/nutritionist says -

He is getting enough calories/calcium/vitamins.
She does not think there is any physiological/medical problem. It's typical 2-year-old stuff.
"Food chaining" is the way to introduce more food. You take foods he likes and match them with new foods that are similar. I.e. Penne (which he eats) with other kinds of noodles. Or "veggie booty" with non-toxic other puffed foods (not cheetoes). Etc. In the meantime, we can just keep feeding him whatever it is he's eating (breakfast stuff, yoghurt, some cottage cheese, some fruits, some snacks, plain penne noodles) so long as we "chain."
Food strategies based on denial/discipline are ill-conceived and likely to have backlashes even if they work. You cannot control eating, sleeping, bowl movements. Don't try. You can, though, influence the decisions they make. This is true for all children.
Most of all - eat with him at least 2 meals a day. Take whatever we are eating and put some of it on his plate/tray. Make food social as he's ready for social; in fact, social is more interesting than just about anything else.
In the end, there are children with Down's who decide on what they will eat, and then they eat only that, potentially forever. If that happens, we'll have to cope most likely. No reason to think that's happening now, though. He's just "food jagging" and exercising power/control in the limited arenas in which he can.
This is all reassuring and calming. We've actually been calm since we gave up and just started feeding him from his current menu. It was just the what happened next that was stressful. Also, in non-food-play, we are to be more assertive in how we deal with his stubbornness. Don't let the fussing, whining, even mild resisting end the play (and lead to "quiet" time) until he really gets obstreperous.

P.S. 4 days in a row successful use of potty post-dinner. It's very encouraging as you can see the comprehension in his eyes.