Oh, HUGS! Sounds Miserable! Have you gotten any answers yet? I'm wondering like MilesMommy that maybe its a sensory issue? If you live in the US, the public school system or county public health should have something like Early Childhood Special Education or Early Intervention program that's free - and they can evaluate your child and help you sort thru things like this if the pediatrician is a dead end. You may need some kind of occupational therapy (it can help with daily living) ECSE can be a wonderful resource to help you sort this all out - they helped us get our daughter's egg allergy diagnosed when she was 15 mos. They can confirm there's a problem and work with doctors and specialists to get the baby back on track and healthy again.
We have lots of experience with constipation/Miralax. For babies - constipation is little pellets or very firm stools - and miralax can help soften. If he's backed up, you may need to give extra miralax until its toothpaste consistency - and then a little more until its runny - to make sure he's cleaned out. our daughter has lots of constipation issues and we do cleanouts every few weeks (she's 5 yrs now, been on it for 3 yrs now).
still look into the reflux, maybe try a white noise in his room (like a fan running or a white noise machine). We've had lots of health issues with our daughter - and you need to keep looking for a pediatrician until you find one that listens to you and helps you figure this out. Keep a log, maybe use a video camera to record these crying sessions, etc. These helped us when diagnosing the egg allergy.