I wouldn't worry. As long as he eats well, and acts "normal" then everything should be fine. Yorkies come in all shapes and sizes, so its hard to say that Gizmo is "underweight." He might just be a smaller size Yorkie. You can always "free feed" him like others have suggested that way he can eat all day long, but it does pose problems for potty training because it doesn't provide that schedule.
Just fyi...Maci is 3lbs 6oz. right now, and she is just now 5 months. Her mom was 5lbs and her dad was 2.5lbs.
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