Just make sure that he gets his fair share of the food. Some piggies are naturally more timid than others. They will come round in their own time, but will never be as forward as their companion - the same as you have bolder and more timid/shy children.
The weight range of newborns that can survive birth is 40g-140g; i.e. the bigger ones are easily double of the smaller pups. This carries through and the weight/size differences between smaller and larger siblings tend to even extend as they grow. One sister pair I had ended up having 500g weight difference in the prime of their lives. It was the younger who ran the group, not her large sister. Both lived to a good old age. Because there is such wide variance between guinea pigs at all ages, weight charts are useless.
You can check the weight/size ratio at any time by checking the rib cage, as explained in this guide here. Weigh once weekly when your piggies are healthy and give your piggies a weekly once-over. As long as your little one is not losing weight, he is fine. Growth and weight gain happen in spurts and eventually fizzle out. My current adult boars vary in weight between 800g and 1500g, and yet all of them are healthy and well in themselves.
The Importance Of Weighing - Ideal Weight / Overweight / Underweight
