Oh, sagarika, what a nightmare for you? Which other foods is your DS allergic to? Mine is allergic to dairy, eggs, wheat, sesame and lentils and this is how I try to get fat and iron into his diet (and the otherwise quite useless dietician we see has at least confirmed this is enough). Apologies for sounding like a complete control-freak:
The weekly meal plan is based around 14 main meals a week, made up of:
red meat x 3
oily fish x 2 (and mostly tinned sardines for the massive calcium content)
chicken or turkey x 2 (turkey has much more iron than chicken, and dark meat much more than white)
white fish x 2
pulses or other vegetarian protein x 5 (baked beans make up at least 2 of these meals, because of the calcium; I also use a lot of quinoa because of high iron content)
Also, for breakfast he gets Readybrek every other morning, and the rest of the time iron-fortified stuff like Wholegrain Rice Krispies or Oatibix.
For fat, I put Oatly cream or Pure spread or olive oil or sunflower oil into pretty much everything: his readybrek, on his rice cakes/crispbreads, mashed potato, mashed squash.
And I give him a vitamin supplement including vits C & D after breakfast (half a dose) and after dinner (the other half). The vit C helps iron absorption, especially when the iron is not in meat and therefore not so easily absorbed by the body (i.e. fortified cereal, vegetarian meals).
HTH.