My lab mix was a little string bean until about 1.5 years. She was skinny like your guy and burned through calories like they were doing out of style (and she is not really very high energy) - just a higher metabolism than most.
She was getting 100%+ more food then she should have needed for her size. Today at the age of 5 (with a buddy dog who keeps her quite active) she still gets about 50% more. It’s just the way she is.
If he is still growing and health issues have been ruled out I would stay the course. Once he stops growing he will fill out. If the vet wants more weight, up the food, add a meal etc.
An interesting note, when my dog stopped growing she filled out QUICKLY. We went from trying to get the weight on to “whoah she’s a chunk” seemingly overnight. I would just be sure to monitor and taper as needed. She ended up having to lose a few pounds to be at her long term ideal weight. Fortunately that has been easy to maintain now that she is older.
