I had my husband watch the Business of Being Born, like others have said. We were only dating at that point (3.5 years before we had a baby), but he pretty much knew from very early in our relationship that that's what would be happening!
Also, though mine was always supportive of it, I think he still felt nervous about it because he didn't quite know what his role would be or how to set up the pool or how to support me since we would be alone for most of it. We went to a local home birth group together and he got to talk to some of the mums there and some midwives, plus taking our NCT class and just learning about how birth worked (pretty much everything the NCT teacher said was beneficial for a safe, easy birth was better done at home than in hospital). But I think the home birth group helped the most. If you can get to know the other home birth mums in your area that way, you might also be able to put him in touch with other dads who might have some advice. I know my husband is now a big supporter of home birth and will tell anyone about it!