Taking over a Mortgage from a Homeowner

[Victor Boyd]

My partner and I have an opportunity to purchase a home in houston.  The owner still owes on the house, but, says he is willing to let us "take over" his mortgage.  

We are not really familiar with the process.  How does it work?  If he still owes $200k on the home and has 20 years left to pay it off, what has to take place for us to "take it over"?   What do we have to do with his bank?

Any help appreciated.