Hi and welcome jen
yeahhhhh for the physio! that referral is at least one thing the paed did get right, as otherwise he doesnt sound all too clued up about the difference between being HM and having HMS...
maybe the physio could write to the paed, explaining that bowel and bladder probs are most likely due to HMS (paed will most likely believe a 'fellow professional' more than "just a parent who read something on a forum"
I'd also ask that physio their opinion on orthotics for your son, and if they can refer him to an OT (occupational therapist) or advise which OT near you might be clued up on HMS- as OT are the ones that tend to deal with hands (so they can advise on/'prescribe' special pens/pencils, supports and exercises for the hands)-
maybe they could include their advice/opinion on that (and also maybe the jaw/grinding teeth probs etc) in letter to paed as well
good luck on your 'journey'- at least you now have one med bod who seems to get HMS and hopefully the physio will take this opportunity to educate a paed on HMS and write to them as then you (and everyone following in the future) will also have a clued up paed-