My son had the same problem with MMA and diper rash. Change the baby more
often. It's own urine/feces has higher acid/toxins in it. Soak baby in
Domboro(not sure of spelling, ask pharmacist.) Put it in tub.

Lastly and most important. Diet related? Possibly diet too lacking in
leucine, isoleucine etc. Have levels checked and adjust.

Hope this helps!