To clarify, if he comes here on a fiance' visa, you MUST marry within 90 days. The K-1 fiance visa is a single entry visa. If he goes back before he gets his green card or his advance parole travel document, he will have to start all over again.
Your best bet as has been suggested, is to get married in the DR (it's much cheaper) and apply for a spousal visa (CR-1). The CR-1 is multiple entry, and is good for 6 months which is plenty of time because he will have his green card in about a month after arriving. It's just an all around better visa and has other benefits as well.
It is NOT true that all other visas are canceled after you apply for a petition for alien relative. If he already has a visa and is in good standing with it, he may use it as long as he doesn't overstay. He may get a hard time at the point of entry, but ultimately if he has not abused the visa they will let him pass, he will just be scrutinized more than usual. I am assuming that he does not already have a visa or you wouldn't be asking these questions. Don't even bother trying to get a tourist visa after you apply for his residency visa because it isn't going to happen. They know he will stay once he gets here.
It is no longer taking a year or more to do the process, married or not. Lately it has been 6-9 months, but of course things can and do change with immigration all the time.
Just one word of caution though. In the last few months on my web site, dominicanstotheusa.com, we have had several guys take off and leave their wives right after they got to the USA. One couple was together for 5 years before they brought him here and when he got here he beat her, and abused her, took some of her things and left. She is currently trying to get him deported. BE CAREFUL. If you don't know this guy, or worse yet if you are a gringa, you have been warned. Please visit our site for more information, the stories are all there.
-Tim