D.B.
I'm confused. You say he is "throwing up" but also that he tends to "hold food in his mouth until he throws up." Do you mean he spits the food out rather than swallow it? Or that he's gagging on it because he can't swallow? Are you sure he is afraid of stomach pain? Or could he have a swallowing disorder? I have a friend whose daughter has Celiac disease as well as a food motility problem, and she stopped eating because food literally caused her pain when she ate gluten (obviously) but also when her stomach remained full. That was worked on through therapy as well as with medical intervention.
A 5 year old who weighs 30 pounds is surely on the pediatrician's radar as having major problems. If he's in school (kindergarten or preschool), then he had to have a physical. It sounds like you don't have a good relationship with the mother - is that true? I would report your observations to the child's pediatrician. Without authorization, the doctor cannot supply info to you about the child, but certainly can take in info you provide. If you have an opportunity to video the child's behaviors around eating, email that video to the doctor. It helps for them to see what is happening, not just hear about it 2nd hand. But if people are giving him crackers and milk because that's what he demands, it's no wonder he's only 30 pounds.
This poor child has so many medical issues, and my heart goes out to him. He needs an advocate.