Denna would do the same thing with her kibble - we just let her graze at leisure. We'd measure out all her meals, so we knew what we were offering, but it would take most of the day for her to finish her meals.
We started adding raw "gravy" to her meals along with some water... that certainly helped her appetite. At that point, I wanted to add raw for extra nutrients, but stick with kibble for convenience and "complete nutrition"... so I just put a few Nature's Variety chicken medallions mushed up on her meals... not enough to really interfere with the digestive systems.
After reading more, we went 100% raw. But, now we have the opposite problem. She'll eat ANY given amount... and we've had to start cutting back portions, because she's gotten a bit "prosperous" about the middle.
Your pup looks perfect to me!
Keep him nice and lean, don't force him to eat more than he needs (it just leads to bigger piles to pick up in the back yard), and I'm sure he'll do great.