Anybody have any ideas how to tighten the loin  up on a dog of 18 months so it dosen't roll when moving in the ring.

a dog will roll if it is overweight less food more exercise ..road walking, hill walking with free exercise.

I have seen dogs that are not over weight rolling so not always fat issue, but often it is little extra under the skin...
I think it depends on the breed, some large dogs definately have the skin roll even if they are correct weight, often more when young and it does get better when maturing...
I don't think there is much you can do if it is an issue of "rolling skin without fat" :) but somebody might have some ideas...
Maybe trying to tighten the muscles by swimming him might help?

18 months is still quite young and in some breeds/lines would still be very immature, maybe the dog just needs time to mature? 

I too have seen dogs 'roll' when moving that aren't fat and are muscular.  If you watch athletes their muscles & skin move too whilst they run, nothing can stay still when it is being exerted.