Formula questions

So my son has been on 4 different formulas he was in nicu for 3 weeks at that time he was having spit up so the typical similac pro advance was not working so they put him the similac for spit up. Kept spitting up and had runs u could tell he was uncomfortable when pooping he looked like he was in pain so we decided after talking to pediatrician to try something gentle on the stomach so we tried the gentlese from was doing good but Wic don’t cover it so they changed it to similac sensitive... soo my question he’s doing good on gentle but it’s for lactose sensitive babies so basically they think he’s allergic to the reg formula. Has any other mom had similar issues?? If so what formula works for u?