I think it's OK. Usually food bags/cans overestimate the amount of food dogs need because they want to sell more food! Plus, it depends on how active he is. I feed Sammy far more than half of what I feed Loki, even though she is less than half his size - she never sits still and he is a couch potato. If he is not hungry and he is growing, playing, drinking water, etc. you shouldn't have anything to worry about! Are you splitting up feedings? My dogs eat 2-3 times a day, depending on our schedule.
You may or may not be able to predict his full grown size. Loki was 3lbs at 13 weeks when we got him and usually you double the 12 week weight. The breeder said he would be 7 or 8 lbs. He is 13 pounds
He is twice as tall as Sammy who is 6lbs. He's a healthy weight for his size, he's just a bit bigger