I leased for 12 years. I was happy for 10.
Know your mileage. It's nice that his work is 10 miles from home, but what about going out for lunch? What about when you meet up for dinner after work? What if you want to take the car? What if his is the "better" car and you take it for road trips? How fast will that mileage add up then?
Know that you will be paying insurance on a brand new car. And you will be paying premiums for a brand new car the WHOLE time you have that car.
Know that if you get a flat and have to replace the tire, you'll be paying for that tire, and when you turn the car in, the tire is theirs now. (super fun when the tires on the car are stupidly expensive and you get to pay for that tire so they match!)
You'll have to baby the car. You don't want any scratches or dents.
When you go to turn it in and you get a new lease on another car, they will almost certainly SAY that they won't charge the turn-in cost, but if you look carefully in your paperwork, it'll be there (under a slightly different name). Hiding. And having interested charged on it.
He really needs to explore why he wants a lease and if it actually makes any sort of sense. For me, I was a young single female and I wanted a SAFE car. I didn't have the money for a down payment and got in on a no-down-payment lease. I didn't have the money for the monthly payment on a loan, but the lease monthly payment amount worked for me. Those were my reasons. They made sense. For 10 years. And then they didn't.
Worst decision I made while leasing was to turn in my first car and get a new one. Bad decision based solely on emotional reasons (sad things had happened while I owned the car...told you, bad decision/reasons). I was the only one who had had that car. It had never been smoked in, I knew the accidents it had been in

rolleyes1), I knew everything about it. I should have paid the highway robbery (that I agreed to when I signed the papers!) to buy it because it's rare to find a car with a single owner that you know personally.
He's got to really think about this. And this will be your money, too, once you are married, or maybe even now. This is a financial decision and should STAY a financial decision. If he's talking you into it, it sounds like emotions are coming into it, and they should not. I tell you from personal experience and the experience of friends of mine...if you're being talked into large financial outlays (whether it's right now or in 4 years when the lease is done, or monthly as you pay the full insurance on that car that isn't even yours) and you're not OK with it, it's not going to get better. Pay attention to what's going on, and when you talk to him again, pay attention to how he deals with that. It's not JUST leasing a car; it's how decisions are made between the two of you.
Best of luck!