The hernia will only cause a problem if a loop of his intestine protrudes through it. Likely, he may have had worms or may have been the runt of the litter. Our pups were 16lbs. at 8 weeks. It's good you are doing the surgery, as an egg sized hernia is large enough to cause serious problems. He will recover and begin gaining weight in earnest. Just make sure you continue to deworm him and keep his shots up to date until he 20 weeks old.