Between my husband and I, we have three cultures, therefore three cuisines. My kids, when little, had sensitive tastebuds, so we cooked bland but a wide variety of foods and mostly managed to get them to eat.
Then when they were teen/tween, we progressively ate spicier. Now my youngest and my husband can eat screaming hot dishes, while my oldest and I stick to mildly spicy.
You are allowed to cook whatever you want for yourself, OP. Indeed, it's a great idea, because maybe when your oldest sees his siblings enjoying whatever it is, he might get out of his picky rut.
And I don't mean to criticize, but how in the world is lentil soup and chickpeas not normal foods? Barring special needs like autism, allergies and intolerances, sometimes I think pickiness is created because the American kid food range is so boring. No wonder they don't eat