Take an honest look at your relationship and make sure this is the right decision for you and your partner. If you're doing this because it just seems like the next logical step then rethink your decision.
Approach asking him to live with you like a proposal. It's not marriage, but it is a big step, so make a big deal of it. Plan a nice evening for the two of you or a special outing.
Have a list of reasons on why you think living with her is a good idea and be prepared to explain them when you ask the question.
Ask in private. People like to propose or ask big questions in very public places but that is generally a bad idea since you really don't know what the person is going to say and you don't won't to embarrass your significant other or yourself.
Set a time frame for the move in to happen. Moving in requires a merging of lives and a lot of compromise so you will need time to pull this off.
Be prepared for rejection. He may say, "No," so prepared for that. However if he says, "Yes," then celebrate, you have just taken a big step in your relationship together.