Well every single BP grows at a different rate I have a I have a 11 months old BP that is about 450 grams, he was the runt of the clutch and grows very slowly. On the other end I have 10 Months old BP who is about 850 grams, I got both of them when they were very young both are fed appropriate size prey once a week yet there is a big difference.

So basically there is no guide, however I would definitely expect a 2 to 3 year old male to be bigger then 600 grams which is why he might be younger then what you were told.

Does he look underweight?

The most important is that as long as your BP is healthy, has a clean bill of health and does not appear underweight there is nothing to worry about.