I would suggest you get the basics out of the way first; cost and size. The cost of the ring will determine quite a bit as well as size (thickness).
Choose a metal that is suitable to your mans life style; some metals are soft and won't stand up to too much wear and tear.
Go with your mans style: strong gold band; simple thin silver band; thick band with a single gem stone (birth stone).
Choose a ring that is comfortable to wear; compliments his finger/hand (skinny band on chubby finger); suits his other jewelry.
Other things to consider: do you want to get it engraved; coordinate with yours; be unique; get it made etc etc.
Choose a ring that looks good and suits his personality.
I have a hub about choosing your wedding ring; you might find a tip or two there.
Congrats and best of luck :)