You ask the mamasan (or the boy) how much to 'off' the boy. Usually starting at about 500 Baht.
This is only the fee to the bar to allow him to finish his shift early.
THEN you negotiate with the boy; variable depending on what you want him to do. Can vary from about 800 to 8,000 Baht.
Take him to your hotel (if allowed) or let him take you to a local short time hotel. Some bars have short time rooms upstairs for a fee of course. Expect to pay about 500 for a short time room
Have fun.