Stuff on lower back

[AdamGB]

You'll likely have tight anterior hip muscles and a weak/inactive posterior chain.

Psoas, adductor and piriformis stretches combined with strengthening the glutes, hamstrings and spinal erectors will be a good start.

No ammount of stretching your back will help long term. That's treating the symptom, not the cause.

[AdamGB]

Also, try and keep a neutral spine when boxing. Ideally any movement of the head or upper torso should be produced from the ankles and hips, not spinal flexion.

[Lefty08]

Stay relaxed and don't let the pain disrupt your focus. Get a good stretch in before and after your workouts. My back gives me problems at times as well because of my last job but you just have to push through it man.

Unless the pain brings you down to your knees, that's my two cents.