I had a horse with a sacro iliac injury.  He had nuclear scintigraphy and it showed bursitis in  the 3rd trochanter region i.e. bad inflammation.  I was advised to lunge him daily to help it, and I turned him away over the winter as well, bringing him back in the spring and commencing with the daily lungeing.  He still needed manipulated every so often, but this gradually reduced until it was only occasionally he needed it.  I know it's not quite the same injury but there could still be some residual inflammation there from the fracture, it does take around 2 years for new bone to be healed completely.  You could try supplementing him with Global Herbs Strongbone and perhaps Movefree Plus, you could email their vet advisors on their website to make sure these would be the correct products - I use these for my warmblood who has been very accident prone over the past year and also has new bone generating in his feet due to having flat pedal bones corrected, there had been some erosion before he had remedial farriery and the bones are beginning to renew.  It took about a year to 18 months for my horse with the sacral iliac injury to get much better, but I think in his case it was the regular exercise which helped.  You could also try lunge work in a pessoa type aid, I used this as well to build strength in the area, and do some lateral work too to aid flexibility.