Baby doesn't tolerate any formula

My baby is 5.5 months old. In the beginning I struggled with the supply and I fed him formula. No problem. After I reached decent supply we ditched formula and he was fed only my milk. But exclusively pumping is so exhausting, I had a goal of feeding him my milk till 6 months. I have a freezer stash so I decided to stop pumping now, feed him my milk and slowly introduce formula. I gave him one bottle of formula, 3 hours after he started vomiting uncontrollably. He vomited all night. When I tried another brand, the same thing happened, vomiting started about 2 hours after feeding, and he became very lethargic. We ended up in the hospital. Doctors suspected CMPA and recommended trying Neocate while we were there. However, even after just 20ml ,mixed with my milk, he had projectile vomiting about an hour later.

Now I’ve been told to keep trying different formulas until we find one he tolerates, but after these severe reactions I am honestly scared to continue experimenting.

Has anyone experienced something similar?What did you do in this situation?

I’m especially worried about what will happen once my breast milk supply runs out.