Hi @candywoo , I can't offer any words of wisdom to your specific situation but my 10-month-old also has global developmental delay and, due to a "lucky" inpatient stay with a chest infection, we were able to get genetic testing started. He has a different trisomy: his is on part of chromosome 12. So, similar to Down's in terms of him having extra genetic material, but a different chromosome and different in its presentation (and, unfortunately for us, its rarity - very few cases so not much info).
What helped us was seeing a GP with a special interest in paediatrics - she started the ball rolling with his referrals once my son was in for his chest infection. Do you have the option for this in your GP practice?
Also, as rubbish as it is not knowing what is wrong (that was the worst part of the journey so far for me), there is definitely something to be said for getting any interventions you can as the condition/syndrome kind of doesn't matter for that, if that makes sense? (In that hypotonia remains hypotonia, whatever the cause.) Portage is a brilliant idea that I see you have already started. Could your health visitor also refer you to occupational therapy or physio? Ours had to do these referrals for us.
It is pricey, but if you can afford even a session or two of private physio, this can give you a really good starting point for doing exercises at home. We see a physio once a month privately but do the exercises daily (and probably ineptly compared to her!!) and my son has made marked progress despite only seeing her for a limited number of sessions.
Wishing you lots of luck with getting the help in place. It is such a mission.
Sorry if that reply is a bit all over the place, my boy had us up in the early hours with another chest infection...!