Chronic long term back pain is often connected to disordered breathing.
In the first picture there is no breath or movement in the back ribs, all the breath pressure is forward and down, keeping the body stuck in extension pattern of anterior pelvic tilt and rib flair.
No support on the exhalation from the internal and external obliques, so they are not helping the pelvis find neutral.
Overtime this has led to an inability of the pelvis to be reciprocal in the gait cycle, which means both sides of his pelvis are ultimately stuck in swing phase of gait and cannot access the stance phase.
It all starts with the breath, re-learning to synchronise the exhale with the obliques/ TA/front rib internal rotation to inhibit the back extensors, while positionally inhibiting the quads and hip flexors by facilitating the lower glutes and upper hamstrings. The awareness will integrate this into his gait so over time with training he can walk/run/breathe with efficency and ease.