Quinton is the trainer for Boxing, located in a roofless brick building, which is bought for $10,000, with the Boxing Gloves being sold for $1000.
The main flaw with Boxing is that all of its damaging abilities requires the use of Boxing Gloves, which makes it difficult to use if the user is a Vampire during daylight. You need to talk to Quinton with $10,000 and his glove in order to unlock the spec.
Quinton: My BOXING skills are sharp..
Player: Please teach me (continue) - Are you sure? (decline
Quinton: First of all, I have lost one of my boxing gloves. And I need $10,000 to start on your boxing career
Player:Am I ready? (continue) - Nevermind. (decline)
If you answer are you sure when he says my boxing skills are sharp:
Quinton: Yeah. Hater.