Snake is not "aggressive"-
....he may be defensive because he is new & scared, & that's normal. You mentioned he is 3 years old...it's also possible he was handled very little by previous owner. A BP that tucks himself into a ball is scared to death of you.
....he's probably hunting & hungry, & your hands are "warm & wiggling" like prey, so he's just being a snake ("strike first & ask questions later"). Up to you to prevent
by making sure he learns to recognize you, but only after he has time to settle in, & only after he is feeding regularly on adequately-sized prey. I think you've been
rushing him: eating reliably comes first, then the handling problem may take care of itself, with a little patience.