
Trust the vets! Obesity is the most common health problem in UK dogs, so most people never see a dog of a healthy weight! And being on the lean side will delay or prevent joint problems. Worm him with Drontal plus every 3 months, and I'm sure he'll be fine.
You should be able to fairly easily feel his ribs when you run a hand along his side, and there should be a distinct "waist" between his ribcage and his hips when you look down on him. If you can SEE his ribs when he's standing, he's too thin.
All my dogs have been permanently hungry! If allowed, one would eat a full bowl of food, go out, throw up and come back to start again! As a rule, labs like their food, so don't go by his appetite. If you do that, you'll end with the more common wobbly lardball!