Does he potentially have any food allergies or intolerances to something in the new stuff? I’m not a parent so I apologise if this oversteps.

Just throwing it out there as that can be sneaky and finding a sensitivity now could save a lot of pain.

Edit: Having a look and apparently it doesn’t hurt them to stay on starter formula until 1 year old, so if he really has too bad a time adjusting you could have the option to revert to what was working. It seems the follow on formula may be higher in iron but I guess in this situation fed is best.

A sibling also had a very relaxed sphincter at the top of their stomach and required thickened formula to help keep it down. If none of that works it’s paediatrician time