well, i reckon i was a hsc (and probably a hsa now!), and i'm also a super-taster. so i can tell you what i don't like if that'll help. if not, stop reading here!
i can't take anything that tastes bitter. so that's veggies that have been overcooked (or even cooked!). brussel sprouts, courgettes, tomatoes (because of the pips) unless they're raw with salt or cooked with seasoning, cabbage, spinach. coffee without sugar, grapefruit. when i was a kid i'd happily eat raw vegetables, but as soon as they were cooked, they were ruined! salads are good, but avoid radiccio and frisee lettuce as they're bitter. iceberg lettuce is good. peas, beans (don't overcook), carrots, sweetcorn, potatoes, roast peppers - probably the only vegetables i actually like! broccoli and cauliflower if not overcooked. tenderstem can be good too, and asparagus. maybe he'll eat asparagus with a nice butter or hollandaise sauce?
don't despair though. my mum used to just make me taste everything and i'm grateful for that now. know i can eat anything that's put in front of me so won't be rude at dinner parties. and i love gourmet food. definitely not stuck on bland things.
just think you have to think in terms of bitterness and try and cut that out. bland is probably working, because it's not bitter.
anyway, that's what i suspect the problem is. sorry for rambling