Hi K.,
Are you breastfeeding or bottle feeding? If you are breastfeeding, I would not worry about food yet. You can breastfeed for up to one year without compromising your son's nutrition. Some sensitivities will go away when the baby is older, others will continue.
If you are bottle feeding him, you will want to very carefully introduce one food at a time (well, this is true of breastfeeding too, but you have more time if you are breastfeeding)
The easiest foods to digest are pureed whole veggies. If you are able, purchase organic veggies such as spinach, carrots, squash (winter or summer), tomatoes, etc. If you have a blender, you can simply blend any of them except the winter squash with some water until it is completely pureed. Then warm it gently and feed it to him. The raw veggies will give him enzymes that cooked veggies do not have, and may help his digestion.
You may try gently cooking these veggies before pureeing them, if you wish. If you cook them, use steam, as it preserves the most nutrients. Do not microwave them.
Another thing to watch out for are the common allergens: wheat, dairy, eggs, strawberries, corn, apples, nuts (strawberries and apples, I believe, are because of the high pesticide content... I have not heard if choosing organic helps). If you are breastfeeding, you will want to check your own diet. My children were allergic to dairy through my milk. I had to eliminate it completely.
I know I wrote a lot. If you would like to learn more, feel free to email me personally at ____@____.com
H.