Hi Yael, DD was on a restricted diet for the same reason as yours, we actually saw a dietician who was very good in explaining it all too us. Best to get a referral to a pediatric dietician as soon as possible because there are chemicals in some fruit and veggies that can set atopic allergies off as well. Also, I understand if you are avoiding dairy you need to avoid soy too (something about the proteins being similar)? Also you will need to delay introducing eggs and in particular egg whites for a bit longer than usual.

Things that we found really useful: porridge made with EBM or rice milk, pureed pear and pureed tinned mango, and i also used to make gravy out of home made stock and add that to her veggies, etc. Once she got to a certain age I could introduce egg yolks which was a blessing, I would make up little vegetable fritters which she loved! This meant I spent a huge amount of time in the kitchen making up tiny little bowls of food, but i soon learnt to cook in batches and freeze little bowls so we had a ready supply of food handy.
Good luck with it, I hope you get a positive result with your son's health!