I'm 5'9" 150#. I was diagnosed with scholiosis in jr. high, had lower back pain when sitting all through hs and college (back when i was 130#). I too tried the standing desk and it didn't help.

Here's what helped -- about 3 years ago I got into squats and dead-lifting. I got them up to 1.5x and 2x body weight, respectively. I didn't gain a whole lot of weight, but my core strength increased tremendously.

Maintaining good posture requires some (~10-30%) flexion of the core/abdominals at all times while sitting. Now, after learning proper form of squats and deadlifts, my body goes into proper sitting form without even thinking.

Back pain is completely gone. squats and dlifts -- highly recommended!

tldr-its not about weight, its about strength