I'm with Chris. If he is never introduced to meat gravy, chicken broth, etc, he won't know what he's missing. You can't miss something you've never had. Also introducing high phe foods at any age is not a good idea, at least in my opinion.
If you want to make him a low protein version of chicken soupyou could use George Washington broth if that is available in your area. If I remember correctly it is not made with real meat products and has no phe. Look for chicken broth seasonings in your local grocery store that say 0 grams of protein and do not have meat products listed in the ingredients, then double check with your dietitian to be sure it is okay.