I just want to echo Bruce, he always has something good to say.

Also, I wanted to point out that males stay pretty long and lean, however, if you bump up his feeding schedule to a weaner rat (60-80 grams) once a week, he will likely put on a good amount of weight and grow.

I've got 2 girls that were very small for their age, they were the same size aas a 6 month old ball python, but over two years old. In 6 months, they have each put on about 600 grams.

I think a 30 gallon size would suit him very well, just keep in mind he will likely prefer a lot of "stuff" to feel completely secure, but then again, he has been in a 50 gallon his whole life, so he is likely used to larger spaces.

Oh, and just to be safe, when he defecates, put it in a ziplock bag and throw it in the fridge. I would get a fecal float done just to make sure he isn't small because of internal parasites eating his meals!