Skinny chihuahua- Eats very little

My chihuahua is 3 years old and he’s always been skinny. 6 pounds to be exact. I took him to the vet once and they didn’t mention anything about it so I never gave it much thought. But really I’ve realized that he’s a finicky eater. He rarely touches his food. He is SUPER hyper and active, acts totally normal and shows no signs of distress. He does have collapsed trachea but it’s not severe. He never finishes his food in one sitting, let alone the entire day sometimes. He does like to get my big dog’s kibble, so we switched him to the same formula (but for little dogs) and he was curious at first but he just gave up on it.

We switched his food 2 times (each done carefully) and he ate for the most part but never really ate a lot. He’s never cared for it. Does anyone else notice this in their dog, specifically chihuahua?

I’m just worried cause he’s literally so skinny. Quite literally looks exactly like the picture I’m attaching.