If he is mildly dysplastic, that's good news!! Look at it this way, had he not been xray'd, would you have known about his hip?? He's not limping, right??
The good news is you DO know about the mild dysplasia, so you will be careful with him. Don't overface him on activities that might challenge the soundness of that hip. Figure out how much activity he can handle and stick to those guidelines. Watch for any unneccessary jumping or use of his hips. Keep him LEAN, and keep him on good supplements. I use Cosequin DS for glucosamine for all my dogs, good hips and bad hips alike. (We've had both over here so I know where you are coming from.) I don't know about adding protein, but your goal is to avoid atrophy of his rear so a good exercise program is in order. Leash walking up hills is a great rear-builder.
Look at it this way, you didn't leave the vets office with a referral to an orthopedic surgeon in your hand, right?? That is great news in itself.
Cheers,
Doofies Mom