A full grown MALE would be closer to 1000g, but females would be closer to 1800 or more. Of course they keep growing a lot of the times, but I think this is a good cut off point.

I still recommend getting a temporary tub. Your adult enclosure sounds nice. Most people don't use flexwatt on a thermostat on the coolside, so I'm guessing ambient temps in your home are a bit low. In a smaller enclosure, you may not have to worry about it as much.

I think it is admirable that you are doing this for you son, just remember you won't know until you have the snake at home and it stops eating. None of us want that. Tub setups are cheap and you can use the flexwatt with thermostat on them as well for the time being, and move it over as it grows. You really don't need anything extra but that tub. And another tub midway through growth. Start with a 15qt and move up to a 28qt around 800-1000g (depends on individual snake), and then once it gets up to a good 1500g I think you could possible move it over. This process could take a couple years, maybe a little less.