11 1/2 month old sensitive belly

My son has had tummy/digestion issues since he started formula. We switched to lactose free milk at 9-10 months. Can’t drink apple juice or fruit juices without having major blowouts. Gatorade is making him poop liquid. Koolaid makes him poop GREEN. Do I just give him milk and water? I don’t get why everything messes with his belly. Anyone’s baby similar or have any advice/help? I do t want to keep taking him to the dr. We were supposed to get into a GI Dr but it’s hard with the covid crap.