Op, your last post sounds very similar to my ds2 and he has dx Joint Hypermobility Syndrome. His legs often just give way underneath him as he's walking. It's mainly his knees and ankles, but sometimes his hips as well.
Frazzled, I took ds2 to the GP about night-time leg pains and because he was complaining that his hips, knees and ankles hurt after exercise - even just walking. He referred to the community paed, who dxd 'lax ligaments' 
and referred to physio. Physio said 'lax ligaments' is a symptom not a dx and told me he fitted the criteria for JHS. We also saw head OT for our area recently who agreed it's JHS.
The only thing is, there hasn't been/isn't any help or support. Physio were hopeless only a short course and admitted they didn't really know what to do for him other than suggest he gets fit. OT were very good at suggesting mobility aids and referred to SN dentist for help with toothbrushing and did go into school to check seating and speak to the SENCO and teacher but that was it.
Ds2 doesn't have orthotics, because according to physio he doesn't have fallen arches, so they wouldn't help. Whereas I did have them as a child, but I have additional problems with rotated thigh bones, not to mention, my physio says she has never met anyone with such low tone in their feet. I have a feeling dd will need orthotics, because she significantly over-pronates and I can see her walking on the inside of her ankles instead of the flat of her foot.
He has 1:1 Pilates lessons, initially working on building and stabilising his core and moving on from there to strengthen muscle groups to reduce the over-movement, iyswim, of his joints.
I am similar to Polter, in that I didn't put two and two together about my own childhood joint issues until ds2 was dxd and felt awful when it dawned on me what was going on for him and how long he'd been suffering (he was 8 when he was dxd).
Neither ds2 or I respond to local anaesthetics and we are all easily bruised - so again similar to Polter.
I now suspect dd (4) may also end up with a dx at some point. She is very hypermobile but it wasn't causing any problems until recently. Unfortunately she is now complaining of sore hands after writing/drawing and having trouble with her ankles.
Ds1 is slightly different. He has ASD and hypotonia which is particularly bad in his core and upper body. He does have hypermobile joints, but doesn't have any pain or suffer sub-luxes and dislocations like ds2, so wouldn't be considered to have JHS.