DS2 was my 'most' hypermobile DC (though 3/4 have dxd Hypermobility).
At 9 mo, he was still as floppy as a newborn and couldn't support his own head. Through perseverance, he finally took his first steps at 3y7mo.
However - there are VERY few babies that are as hypermobile as DS2! He is the most severe my Paed had seen in 20+ years of practice. Ditto the Physio & OT.
Is your DS2 rolling over? Sitting? Crawling?
A good place to start would be to ask for a referral to Orthotics. The Orthotist will decide whether Piedro boots, orthotic shoe inserts or splints would be the best thing to help your DS2. I GREATLY recommend orthotics for helping hypermobile DC's with posture and ability to weight bear, support themselves and walk.
The way I eventually got MY DS2 walking was after he was crawling, holding him up under his armpits (never by hands in the air, as his shoulders dislocate too easily), put his feet on top of my feet, and step, step, step. For hours. Every day. For a year.
I can just remember back then, how I used to cry when he was in bed, thinking he would never walk.
Fast forward a few years, and he's now 9y3mo. And doesn't stop running!
(Ok, he falls a lot still, can't yet pedal a trike, and can't do monkey bars because his shoulders still dislocate, but he runs! And climbs! And jumps! that one took him until he was 7yo before he actually left the ground... )
Your DS2 sounds like he is more like my DD and my DS3. DD walked at 19 months, and DS3 walked at 14 months. Mind you, DS3 still isn't safe walking around outside, he falls too often and he's been walking for 12 months now. I'm hoping his orthotic shoe inserts help. They've made a visible difference to his posture as soon as they went in today.