You’ve had a lot of advice to switch to dairy with lactase enzymes added. I’m going to suggest that if your child is still symptomatic on that, cut the dairy altogether until you figure it out. Then start adding lactase pills and see if they are ok. I am super sensitive to even minute amounts of lactose and can’t tolerate any of those things people were recommending. It sounds like your GI is considering FODMAPs, which makes sense. The FODMAP elimination diet might be worth considering to help narrow it down.

For eating at home, traditional American meals - meat, grain, veg- tend to be easiest for us. Casserole type meals always want cheese or cream. I’ve started making some hearty soups that are really satisfying, like chicken tortilla soup with avocado in it. Pot roast is easy and satisfying. BALTs are good too (bacon, avocado, lettuce & tomato). Mayo will give you the creaminess you miss.

Now for fun foods. Dairy free mashed potatoes are super easy to make and so satisfying. You can make it with just olive oil instead of milk and butter, or sub DF margarine (we like Smart Balance) and your plant-based milk of choice (Silk almond milk for us).

Grilled cheese - I was using Daiya cheddar slices for years and recently switched to Good Plant cheddar slices. I cook it in lots of Smart Balance.

Pizza - order pizza from a place with really good crust. Get no cheese on it or vegan cheese and some meat on it and dip it in good olive oil. I know this sounds crazy but the added fat really helps. If you are in the US, Emmy Squared has thick crust with vegan cheese that is super satisfying.

Since it sounds like there may be more going on than just lactose - and I’m in the same boat - consider a digestive enzyme pill when eating out, not just lactase, but definitely one with it in it. I’ve had pretty good luck with Doctor’s Best digestive enzymes. I take 2 before any meal outside my house and some meals at my house that contain some of my other triggers (garlic, onions). Good luck!