My brother, 19 has autism + gerd, non-verbal. past 6 days, he's been lethargic, not eating, which is abnormal bc he loves food, screaming in pain, pointing to the bottom of his sternum. er doctor said it's constipation, but he pooped + still in pain?

His main treating doc will be best to at least coordinate locally as it is likely that there is a different condition causing his symptoms. Alternatively a local urgent care can be used.

Take him back to the ER, they need to look for another explanation and they will. He at least has a gastritis. Other possibilities just become more serious from there. A week of severe abdominal pain without improvement is an emergency and his autism only makes a serious cause more likely since he can't communicate specific symptoms and they often tolerate severe pain better than lesser stimuli.