I'm going to second this. And third it. Once a boa hits 12 months old it should be on a 10-14 day schedule. The one you purchased should be several years old and eating every four weeks at most - some of my males are on 5-6 week schedules, eating just one 12-16 oz rabbit since large+ rats have a high fat content.
What is this guy's body shape like? Is he squared up like [] or round like () on his sides? Hopefully his back end doesn't look like a stack of tires.
And be prepared for him to beg. He will tell you he's huuuuuuuuuuuuuuuuungryyyyyyyyyyy. He's staaaaaaaaaaaaaaaaarviiiiiiiiiiiiiiiiiiiiing. He's waaaaaaaaaaaaasting aaaaaaaaaaawaaaaaaaaaaaaay from lack of food. IGNORE HIM. Boas will eat themselves to death if you let them.
He won't be doing much moving around in a 4x2 so I would also suggest that after he has a few weeks to settle in with you, get him out and hold him, take him in the yard on nice days, etc. Keep him active. It's a pain while they're in QT since you have to go shower right after but it will be worth it, especially if he's started packing on extra weight due to being overfed.