I feel for you op but doubt I can be of much help.
My ds is 4 and has suffered from constipation due to withholding. When he was toilet training he could hold wee for a full day too, thankfully that passed in a matter of weeks.
We've only had a breakthrough on the stool front thanks to being referred to a marvellous consultant. He was patient and literally at the end of the phone whenever I needed to chat.
We were prescribed movicol(which da was on for months prior anyway) at the maximum dose for a week- he dealt with it really well. Much better than we hoped. He was also prescribed sodium pico sulphate, is this something your ds has had.
I think the pico has made a huge difference, our consultant said it gave the bowel "some umph" we first visited him in October and ds has not needed movicol since that initial week.
He is emptying his bowels every other day at least now and 6 not he ago id not have thought that was possible.
Ours is positive but in different circumstances sadly, hopefully it can provide some hope for you though.
Is your ds under a consultant?