Borrowing money is never a easy thing to do for anyone, the reality is that it puts strain on any relationship if something happens to where you can not fulfill the agreement made on the loan.
The good news is that since you have been together as long as you have asking for a loan or to borrow money for emergency's isn't a real big deal as long as you understand that you have to offer to pay it back and make sure you do so.
The best route is to simply sit him down and explain your situation. Let him know why you need the money and how much you need. Explain to him that you hate to ask him but have no other option. Make sure you let him know when the money will be paid back and how (Such as payment plan etc).
Make sure you look at your other options first, make sure there is no other route you can go before asking him. The fact is borrowing money from someone other then family should never be done unless you have no other options.