Trunk Instability

Hi,
Iam treating an Elderly Gentleman whose mobility has gradually reduced in the last 2 years.I found his pelvic girdle to be fixed with negligible movements
he presents with ,
Decresed trunk stability posteriorly.
Inability to stand upright even with full support due to a stooped pelvic girdle.
Gradual loss of weight and decreased muscle mass around pelvis and abdomen.
Apart from this there is no loss in muscle power and has grade 5 in all 4 limbs.
I wonder what actually is wrong here,there is no formal diagnosis until now.
Any suggestions would be very much Appreciated