Each snake grows at their own rate, especially if you have to deal with picky eaters or those that go on strike. There really is no "set" weight for snakes.

However with that being said, 146 grams for a 6 month old does seem a little low, I have 6 month olds that are 450+ grams.

If anything you can up his feeding schedule so he can start putting on some weight, so instead of once every seven days, feed him once every five.