Spring is Spargel season in Germany, and it's usually prepared steamed or roasted. Of course, it often comes with a heaping dose of Hollandaise sauce, but you can probably request to have it withheld.
If you're driving on the Autobahn network, most of the restaurants usually have an impressive salad bar. Not the crappy iceberg lettuce kinds you see in the US, but decent prepared salads featuring various kinds of vegetables.
If he likes the tastes of preserved fish (I've found many Americans don't), look for the Nordsee fastfood chain. They have small take-away sandwiches made with lax, maatjes, and pickled herring. Good low calorie source of protein.
Although usually not found in the main areas were tourists congregate, you can find plenty of small lunch restaurants that offer take-away salads.
"And don't forget that yummy cucumber salad found in so many places - wish I had a good recipe for that!" It's pretty easy to make, but it's high in salt. You peel the cucumbers and slice them thin with a mandoline. Sprinkle the slices with salt, and let it sit for an hour. Drain off the excess water, add the sliced onions, a little sugar, pepper, dill, and a simple oil and vinegar dressing. There's also a version that uses a cream-based dressing.
