How old are you?
Coming from a Catholic home, I know that our belief is to get married then have children. But it doesn't always happen that way and as difficult as it is for a parent to comprehend and accept-- you are their daughter. I know that for those with a strong faith in Catholicism it may seem like the end of the world, their daughter is having a baby out of wed-lock.
The good thing is that you have decided to tell them now, rather than wait it out. The sooner you get it out of the way, the better. As far as how you should do it, well, you know your parents best and although I can give you advice I'm not sure how well it may work for your situation.
Here is what I can suggest: Judging by your father's health-- telling your mom first may not be such a bad idea. I am assuming that she has been married to your father for a while now and that she would probably know what the best way to break the news to him is. However, if you think it is best to tell them together-- make sure they are home, there are no visitors and that you have a plan. Know what you are going to do when they ask "so, what now? are you going to get married?" Every parent wants to know where their child is headed now that there is a baby on the way and if you and your boy friend can answer those questions then it may help them understand. Also, do you think it's best for your boyfriend to be there or if it helps to have your parents cool down first before they see him? That's another thing to consider though I think it's commendable that he is willing to go to your house and talk to them personally.
It seems like the hardest thing in the world to do right now but it's the right thing to do and in the end you will feel much better about it.