? Unknown pathology

Some ideas please:

6 year old boy with a 12 month history of progressive contractures of fingers, hands, wrists, forearms, ankles, knees. He is contracted now to the point of disability.
Neurologically in tact, sharp blunt distorted over anterior forearm, everything sharp. Light touch fine.
Reflexes normal
Fingers~30deg contracted at all distal joints
Thumb~20deg abduction/extension
Wrist~Neutral only, no extension
Supination~Neutral only, no extension
Elbow~normal
Shoulder~Normal
Ankles~90deg only
Knee angle with hip at 90~40deg
Knee ext~Full
Hip~NAD

Cannot sit on floor x legged or long sit

As an infant crawled in full wrist ext, photo of holding ball 12 months ago with hands fully extended.

What could cause this. I am asking for a referral to our connective tissue clinic but some ideas regarding pathology and progression would be great as I have been asked to address the contractures until specialist review which can take months.

Thanks a million.