There's no set growth rate and all snakes are different, but if he's a consistent eater on appropriately sized prey, ~50-100 grams a month should be a general ball park figure of growth.

For example, I had two females born within days of each other. They were both fed the same prey sizes and on the same feed schedule. One girl outgrew the other by several hundred grams within a few months. By time they were a year old, one was over 1000 grams and the other was 500-600g.