My dog's coat is very dry especially on his rump. He is fed Pro Plan Lamb and Rice with tin tripe and suggestions.

Is it just the coat,or is the skin dry too?

I would change the food.  Doesn't sound like its agreeing with him.  I use RC and I add oily fish into it too.  My dogs have excellent coats.  Try oiling the coat too.  Don't know what breed it is though so that might not be an option.

It looks like just his coat. He is an australian shepherd. What food would you suggest cannot give this breed high protein as it makes them too hippy.

Don't they get a harsh top coat at the rump naturally? I know a rough collie like this and apparently it is to do with being bred for the harsh weather.  Shelties too. 
If it's bugging you, smear a little olive oil on your hands and pat down the coat or use a really good conditioner when you wash him or a leave in conditioner.  My little cocker has a bit of a flyaway coat so I use pantene smooth and sleek on her.  I leave it in for a while then wash it all out, then after I towel dry her, I put a tiny bit on my fingers and rub into the section that's the worst.  You also get leave-in conditioning sprays that are good for dry coats. 
I would do this if you are otherwise happy with the food.

Sea breeze oil from Plush Puppy is also a good light oil that comes in a spray. I use it sometimes to stop the coat breaking on one of my OES.

I feed mine Autarky Salmon and Rice - 24% protein so a little lower than the pro plan, but 35% salmon.  Everyone's coats are brilliant, not dry at all, lush and shiny.

Have you tried supplementing his diet with salmon oil?  Very healthy and a gentle first option.

I used to use Pro Plan Salmon and Rice had problems that's why I changed.

Have just ordered some samples of the Autarky to see if it suits.