caging option

Would it be possible to keep a young 15-18" boa in half of a t8 ap cage?? Basically a 2x2x12

Sure but depending how aggressive you feed it, don't expect that size to last over a year. The rule of thumb I use is add up the length + width and that is the max length of the boa. You want the boa to be able to stretch out fully using the length and width. So for example, a 4x2 cage would comfortably house a 6' boa. Of course adding a shelf or perches etc will increase the overall floor space but i'm rambling...bottom line is yes.

Yes he will eventually have the 4 foot cage to himself. I am only putting the divider in since he will be only 20" long when i get him. This cage should be sufficient for his whole life.

Oh if you are just dividing it because hes small, don't do that. He will be fine in T8 as a baby. I put Rosey in a 48x24x14 Proline when she was about 15" and she was fine. Just give your little guy a couple hides and maybe throw in a couple paper towel tubes and a silk plant from craft stores like Michaels and he will be happy. Boas aren't like BPs, they are much more outgoing and not really that shy honestly. At least my girls aren't but then again, Dottie the BP isn't very shy anymore either. She comes out at night and just lays her whole fatness out across the front of the cage and rests her head on her stomach lol.

Oh, I didn't know I could use the whole 4ft cage at this time since he is so young. I think he is at least 6-7 months old. I am told he is about 20" long.

He is currently being kept in a tub at the breeders. I am waiting for my 4ft cage to be delivered but would love to move him directly into that since it has an RHP for heat - which I prefer to belly heat.

I just don't want to stress the little guy out.

Nah, he'll be fine in a full 4' cage. Boas are pretty brave compared to say a BP lol. just make sure he has a couple hides at least and he'll do fine.