Congrats on tracking down a cb baby!
He sounds like he'll be the same size as my cb male was when I got him. If I remember correctly, I got Rigel in late September. He wasn't sexable at that size, but I had a hint of the sex by December since the scales surrounding the pores on the underside of his hind legs were very well developed, and clearly visible to the naked eye. By the end of March, there was no doubt, the pore "popped".

Rigel was a bit underweight when I got him, maybe in part due to his cross country trip from Florida to California. I set him up in a 20 gallon and covered all the sides with paper so he wouldn't be disturbed and had lots of rocks he could hide under / between (just make sure they can't shift!). Tight hiding places seem to be really important for them to feel secure. Rigel would squeeze down as flat as a pancake to get into the 1 cm gap between the two rocks where he slept.
I put Rigel on playsand right away. I kept his food in a bowl and placed the bowl on a flat rock to minimize the chances of him ingesting the sand. Flowers are a real favorite, especially dandelions, light colored pansies, and nasturtiums. Dadelion greens, plantain, nasturtium greens, mint (Rigel likes it, but Gracie doesn't), are some of my staple greens. Rigel also likes worms (mealworms & waxwoms). I had originally got the waxworms for Gracie but she wouldn't touch them so I decided I'd offer one to Rigel. He crawled right up to my hand and took it. Hand feeding is great because it gets them to associate your hand with good things like food, and they become more willing to be handled and less stressed by it. I wouldn't push the handling (other than offering tasty morsels by hand)until you're sure he's eating well. You can offer water to be safe, but many chucks won't drink. You can make sure he's well hydrated by washing the greens and putting them into the bowl wet. The T-rex iguana dust is the only supplement my guys find palatable, I sprinkle it on their dandelion flowers about once a week (even dusted dandelions are too tasty to resist).
I had the basking temp at a little over 100 in the 20 gallon. By about 2 and a half weeks, Rigel had just about doubled in weight and I moved him into the 100 gallon with Gracie. In the 100 gallon, the basking temp at the elevated basking site is around 120 (I was initially told 130 from the supplier who said that that was the temp recommended by the breeder, but I felt this was too high). There is a lower wattage bulb creating a second basking site towards the middle of the tank that gets to be around 100. The cool side fluctates between the 70's and 80's depending on the room temp. I use a mercury vapor bulb on the warm end and a Reptisun flourescent on the cool end for UV.
Congrats again on your new addition!
Alice